Niagara Power Project FERC No. 2216
DESCRIBE NIAGARA RIVER AQUATIC AND TERRESTRIAL HABITAT
BETWEEN THE NYPA INTAKES AND THE NYPA TAILRACE (U.S. SIDE)
HTML Format. Text only.
Prepared
for: New York
Power Authority
Prepared
by: Aquatic Science Associates, Inc. and E/PRO Engineering & Environmental
Consulting, LLC
August
2005
Copyright
© 2005 New York Power Authority
_________________________________________
EXECUTIVE SUMMARY
The New York Power Authority (NYPA) is engaged in the
relicensing of the Niagara Power Project (NPP) in Lewiston,
Niagara County, New York.
This study was conducted as part of the relicensing process. The objectives were to: 1) Describe the
aquatic and riparian habitats on the U.S.
side of the Niagara River in the area between
the NYPA intakes and the NYPA tailrace at flows of 50,000 and 100,000 cfs to
assess the effects of water level and flow fluctuations, and 2) identify and
describe factors that influence these habitats.
The Treaty Between
Canada and the United States of America Concerning the Diversion of the Niagara
River, Oct. 10 1950, 1 U.S.T. 694 (Treaty) specifies the seasonal flow
regime over the Niagara Falls and through the Niagara Gorge. The Treaty establishes a seven-month tourist
season from April through October, and a five-month non-tourist season from
November through March. The Treaty
requires a minimum flow of at least 100,000 cfs over the falls during tourist
season hours from 8:00 AM to 10:00 PM EDST during the period April 1 to
September 15, and 8:00 AM to 8:00 PM from September 16 to October 31. At all other times during the tourist season,
as well as during the entire non-tourist season, the minimum flow requirement
is 50,000 cfs. Flows are sometimes
greater due to storm surges from Lake Erie,
ice management, wind conditions, and regional precipitation patterns that
affect lake levels (URS et al. 2005).
The study methodology involved compiling and reviewing
existing information about aquatic and terrestrial habitats in the
investigation area and collecting additional information during field
surveys. The field surveys included
aerial surveys that were conducted at approximately 50,000 and 100,000 cfs, and
documented with video. Ground surveys
were also conducted throughout the entire gorge to delineate habitat reaches,
measure areas affected by flow changes, document terrestrial communities,
locate and assess waste discharges, and observe the effect of mist and ice at
the falls. In addition, ground surveys
were documented with video.
Two major reaches were delineated to characterize the
aquatic habitats of the upper Niagara River from the intakes to the Niagara Falls. These are the Chippawa-Grass Island Pool
(reach 1) and the Cascade Rapids (reach 2).
The
Chippawa-Grass Island Pool is up to 12 feet deep, with water velocity up to two
feet per second and a man-made shoreline composed of fill and rip-rap. The Cascade Rapids are a shallow, high-energy
habitat with boulder and ledge substrates.
Since these two reaches are extremely wide, increasing flows
cause relatively small increases in water levels. The Cascade Rapids have a median daily water
level change of 1.0 feet during the tourist season, and 0.3 feet during the
non-tourist season, as measured just upstream of the American
Falls. This daily flow
change has relatively little effect on near shore aquatic habitats, with the
exception of the Three Sisters Islands
where the channel adjacent to Goat Island is
dewatered when the flow drops to 50,000 cfs.
In the Chippawa-Grass Island Pool, the daily median water level change
is 1.5 feet in the tourist season and 0.5 during the non-tourist season as
measured at the Material Dock Gauge.
Water level changes in the Chippawa-Grass Island Pool, although
relatively small, are quite frequent due to water level management for
hydroelectric water withdrawals and compliance with the Treaty and IJC
directives. Near shore habitats in the
Chippawa-Grass Island Pool are composed of fill areas with rip-rap or cobble
substrates.
Five aquatic habitat reaches were delineated in the
Niagara Gorge: Maid of the Mist Pool (reach 3), Whirlpool Gorge (reach 4),
Lower Gorge Run (reach 5), Foster Rapids (reach 6), and Tailrace Run (reach
7). These five aquatic habitat reaches
include pools, runs and rapids. Pools
are deep and slow moving with very little gradient while rapids are relatively
shallow and fast moving with a steep gradient and runs have intermediate
characteristics. In addition to these
inherent hydraulic differences, there are four hydraulic controls in the
Niagara Gorge that also affect how these reaches respond to flow changes. The Maid of the Mist Pool is a very deep
(maximum depth over 200 feet) pool, with low water velocity, steep shorelines,
and ledge/boulder substrates. Water
levels in the Maid of the Mist Pool are affected by a hydraulic control just
downstream of the Whirlpool
Bridge. As a result of this constriction, the flow
change from 50,000 to 100,000 increases the water height about 11 feet. This is documented by data from the Ashland Avenue
gauge, which is located in the Maid of the Mist Pool – the median daily water
level change in the tourist season is about 11.1 feet. During the non-tourist season, when flows are
more stable, the daily median water level change is 2.9 feet. Although tourist season daily water
level changes are much greater than daily changes in the non-tourist season,
the seasonal fluctuation range is actually greater during the
non-tourist season due to storm surges from Lake Erie. Winter storms on Lake Erie cause surges
through the Niagara River that can significantly increase flow over Niagara Falls and through
the Niagara Gorge.
The Lower Gorge Run (reach 5) is a wide reach with
moderate water velocity and steep shorelines composed of cobbles and
boulders. Water level fluctuations
appear to be similar in the Lower Gorge Run, as in the Maid of the Mist Pool
because there is also a hydraulic control at the end of the Lower Gorge
Run. When the flow increases from 50,000
to 100,000, this hydraulic control creates a large standing wave that backs up
the water in the Lower Gorge Run.
The Whirlpool Gorge (reach 4) includes four features that
were formed by the St. David Gorge, which is thought to have been eroded during
the middle of the Wisconsinian Glaciation and filled at the end of the
Wisconsinian Glaciation (Wachtmeister 1997). These sub-reaches are known as the Whirlpool
Rapids (4A), the Eddy (4B), Little Niagara Falls (4C) and the Whirlpool
(4D). Hydraulic controls affect water
level fluctuations in the Eddy (reach 4B) and the Whirlpool (reach 4D). The sandstone layer that is exposed at the
downstream end of the Eddy constricts the flow and creates Little Niagara
Falls. The same sandstone layer also
affects flow from the Whirlpool, although this is a much smaller hydraulic
control. As a result, there are large
water level changes in the Eddy and the Whirlpool. Flow changes also affect the direction of
flow in the Whirlpool – at low flows (about 50,000 cfs) the direction of flow
is clockwise around the margin of the pool, while at high flows (about 100,000
cfs) there is a counterclockwise flow and a powerful whirlpool. The two rapids reaches, Whirlpool Rapids
(reach 4A) and Foster Rapids (reach 6), respond to increased flows with less of
an increase in water level than the other Niagara Gorge habitats since the
steeper gradient of these reaches increases the water velocity, more than water
height, relative to the other reaches.
Potential fish spawning was evaluated at sites that could
be dewatered when water levels are low.
One of those sites is a large rocky peninsula that extends out into the
lower portion of Fosters Rapids (reach 6).
This area was identified by NYSDEC.
This site is not suitable for spawning by salmonids or sturgeon since it
lacks the gravel required by salmonids and is too shallow for sturgeon, even at
high flows. However, gravel was present
along the shoreline just downstream of this peninsula in backwater and this
small gravel deposit was used by spawning steelhead in the spring of 2003. This area has a shallow slope and a portion
of it could become unusable to fish at low water levels. Fosters Rapids appears to have abundant
sturgeon spawning habitat, particularly at the downstream end where numerous
boulders and rubble substrates are found in depths of about 6-15 feet. Hughes (2002)
studied the movements of sturgeon in the lower Niagara River, however, the
Niagara Gorge was not included in the study so it is not known whether sturgeon
actually spawn in Foster Rapids.
Eight major terrestrial habitat types were observed and
mapped in the investigation area. These
were mapped and classified at the natural community and land-use covertype
levels. These habitat types occur
throughout the investigation area and are composed of a variety of native,
non-native, invasive, and horticultural plant species.
As determined through literature review and observations
while conducting fieldwork for this study, these habitats are influenced by a
number of factors. These include water
level and flow fluctuations, ice formation and accumulation, groundwater
seepage, surface runoff, and sewer and storm drains, recreational activities,
and the introduction of invasive and horticultural plants. The influence of water level and flow
fluctuations on terrestrial habitat is minimal because most of this habitat is
located above the influence of fluctuating water levels. Small, fringe areas of riparian wetland
vegetation associated with the calcareous talus slope woodland and limestone
woodland community type experience daily water level fluctuations during the
growing season. However, these areas
appear to be relatively unaffected by water level and flow fluctuations as they
are composed of species that are tolerant of daily root zone saturation and/or
inundation, and discernible changes in species composition and shifting of
vegetation zones either landward or waterward does not occur from year to
year. At the time of the field surveys,
the observed difference between the amount of falls-generated mist produced at
the 50,000 and 100,000 cfs flows was minimal.
Factors that may influence the size and distribution of mist clouds at
both flow regimes include temperature, weather conditions, relative humidity,
dew point, and wind speed and direction.
Ice formation and accumulation, and ice loading on vegetation appear to
be factors that influence terrestrial habitat from the southern end of Goat Island to the Whirlpool Rapids. Mist freezes on vegetation and ice
accumulates in the Maid of the Mist Pool causing large ice dams. The ice scours nearshore areas preventing
soil formation and accumulation and removing vegetation. This is an annual natural event that affects
the growth and distribution of plants in near shore areas in the vicinity of
the falls. Prior to utilization of an
ice boom, ice accumulation in the Niagara Gorge was much more severe. The ice boom mitigates these effects by
preventing large ice floes from entering the upper river and reducing ice
accumulation in the lower river.
Observations during the field surveys and information from
literature suggest that groundwater seeps, surface runoff, and sewer drains can
influence terrestrial habitat by introducing salt-laden runoff (from winter
road maintenance) into the gorge.
Stormwater runoff from city streets and parking lots may introduce
various chemicals and petroleum products into the gorge as well. Salt-laden runoff can encourage the growth of
salt-tolerant invasive plant species, while salt and chemicals in runoff can
stress and possibly kill vegetation.
Recreation is thought to be a major factor that influences
terrestrial habitat. Hikers and bikers
traveling on both authorized and unauthorized trails can trample plants, cause
soils erosion and loss, and cause soil compaction (Riveredge
2005). The collection of plant
specimens can lead to the eradication of some species while encouraging the
growth of others.
Invasive plants and
planting of horticultural species can influence vegetation in the investigation
area. The planting of non-native trees
and shrubs on the edges or rim of both sides of the Niagara
gorge influence the plant composition of natural communities. Specifically, it is thought that the planting
of alien invasive trees and shrubs to landscape public parks on both sides of
the gorge can influence natural communities in the gorge itself. However, the planting of introduced Eurasian
species in Buffalo occurred as early as 1886 and
many non-native plant species were introduced into western New
York and adjacent Canada
by European settlers and occur throughout this region today. Some of these alien invasive species have
displaced or have the potential to displace native plant species.
1.0
INTRODUCTION
The New York Power Authority (NYPA) is engaged in the
relicensing of the Niagara Power Project (NPP) in Lewiston,
Niagara County, New York.
The present operating license of the plant expires in August 2007. In preparation for the relicensing of the
Niagara Project, NYPA is assembling information related to the ecological,
engineering, recreational, cultural, and socioeconomic aspects of the
Project. As part of this effort, Aquatic
Science Associates, Inc. (ASA) and E/PRO Engineering and Environmental Consulting,
LLC (E/PRO) investigated habitats and environmental conditions from the intakes
to the tailrace of the NPP. The
investigation area for this study includes U.S.
waters of the mainstem Niagara River and associated terrestrial habitats
between the Niagara Power Project twin intakes upstream of Niagara Falls to the Robert Moses Power Plant
tailrace in the lower river, and Artpark (Figure 1.0-1).
The 1,880-MW (firm power output) NPP is one of the largest
non-federal hydroelectric facilities in North America. The Project was licensed to the Power
Authority of the State of New York
(now the New York Power Authority) in 1957.
Construction of the Project began in 1958, and electricity was first
produced in 1961.
The Project has several components. Twin intakes are located approximately 2.6
miles above Niagara Falls. Water entering these intakes is routed around
the Falls via two large underground conduits to a
forebay, lying on an east-west axis about 4 miles downstream of the Falls. The
forebay is located on the east bank of the Niagara River. At the west end of the forebay, between the
forebay itself and the river, is the Robert Moses Niagara Power Plant, NYPA’s
main generating plant at Niagara. This plant has 13 turbines that generate
electricity from water stored in the forebay.
Head is approximately 300 feet.
At the east end of the forebay is the Lewiston Pump Generating Plant. Under non-peak-usage conditions (i.e., at
night and on weekends), water is pumped from the forebay via the plant’s 12
pumps/generators into the Lewiston Reservoir, which lies east of the
plant. During peak usage conditions
(i.e., daytime Monday through Friday), the pumps are reversed for use as generators,
and water is allowed to flow back through the plant, producing
electricity. The forebay, therefore,
serves as headwater for the Robert Moses plant and tailwater for the Lewiston
Plant. South of the forebay is a
switchyard, which serves as the electrical interface between the Project and
the interstate transmission grid operated by the New
York Independent System Operator.
The objectives of the study were derived from issues
raised by various stakeholders. The
objectives listed in the scope of services are as follows:
·
Describe the aquatic and riparian
habitats on the U.S. side of the Niagara River in the area between the NYPA
intakes and the NYPA tailrace at flows of 50,000 and 100,000 cfs to assess the effects of water level and flow
fluctuations; and
·
Describe the
past and present terrestrial habitat in the context of cumulative effects.
The Niagara River drainage area includes four of the five Great Lakes, an area of approximately 263,700 square
miles. The difference in surface elevations between Lake Erie and Lake Ontario
is about 326 feet, half of this occurring at Niagara Falls. The Niagara River consists of two major
reaches; the upper Niagara River and the lower Niagara
River. This report covers the upper Niagara River from the NPP
water intakes to Niagara Falls, and the lower
Niagara River from Niagara Falls
to the end of the Niagara Gorge, just downstream of the tailrace of the NPP.
The upper Niagara River extends about 22 miles from Lake
Erie to the Cascades Rapids, which begin 0.6 miles upstream of the Horseshoe Falls (Canadian side of river). From Lake Erie to Strawberry
Island, a distance of approximately 5
miles, the channel width is greatest at the river’s head (9,000 feet.) and
least at Squaw Island,
just downstream of the Peace
Bridge (1,500
feet.). Average channel velocities are
approximately 5 to 9 feet per second (fps) in the vicinity of the Peace Bridge. Between Squaw and Strawberry Islands,
the river width is approximately 2,000 feet, with average channel velocities on
the order of 4 to 5 fps.
At Grand Island, just
downstream of Strawberry
Island, the river divides
into the west channel, known as the Canadian or Chippawa Channel, and the east
channel, known as the American or Tonawanda Channel. The Chippawa Channel, approximately 11 miles
long, varies in width from 2,000 to 4,000 feet.
Average channel velocity is 2-3 fps.
The Chippawa Channel carries approximately 58% of total river flow. The
15-mile-long Tonawanda Channel varies in width from 1,500 to 2,000 feet
upstream of Tonawanda
Island. Downstream of this island the channel varies
in width from 1,500 to 4,000 feet, with average channel velocities of 2-3
fps. At the downstream end of Grand Island (i.e., the
north end), the channels unite to form the 3 mile-long Chippawa-Grass Island
Pool, at the lower end of which is the International Niagara Control
Structure. This linear structure, with
18 sluice gates for control of flow over Niagara
Falls, extends perpendicularly from the Canadian
shoreline to the approximate midpoint of the river. The Falls is located
about 4,500 feet downstream of the International Niagara Control Structure.
The fall (i.e., change in elevation) from Lake Erie to the Chippawa-Grass Island Pool is
approximately 9 feet. Below the
International Niagara Control Structure, the river falls 50 feet through the
Cascade Rapids, which are divided into two channels by Goat
Island. These channels
convey the flow to the brink of the Canadian Falls (also known as the Horseshoe
Falls) on one side and the American Falls on
the other. At this point the river drops approximately 167 feet, on the
American side falling on a sizable volume of talus, or rock debris,
that has accumulated at the foot of the precipice.
Below Niagara Falls (i.e., in the lower Niagara River),
the river runs through the narrow and spectacular Niagara gorge seven miles
from the Falls to the foot of the Niagara escarpment at Lewiston, New
York. The upper portion of this reach,
which is navigable, extends from the base of the Falls
to the Whirlpool Rapids, which are not navigable. The fall through this upper reach, known as
the Maid of the Mist Pool, is approximately 5 feet. In the Whirlpool Rapids, the water surface
elevation drops approximately 50 feet over the course of a mile, and velocities
can reach 30 fps. At the Whirlpool, a
1,700-foot-long, 1,200-foot-wide, 125-foot-deep basin downstream of the rapids,
the river bends nearly 90 degrees to the northeast. Below this point the river drops another 40
feet through the Foster Rapids. It
emerges from the gorge at Lewiston, New York, subsequently dropping another 5 feet to Lake Ontario,
and widening to 2,000 feet. The Niagara
River is navigable from the mouth at Lake
Ontario to just upstream
of the NPP tailrace by conventional watercraft, and upstream to the Whirlpool
by specialized watercraft (Stantec et al. 2005).
Figure 1.0-1
Investigation Area
[NIP – General Location Maps]
2.0
FORMATION
OF THE NIAGARA GORGE
The Niagara River is
somewhat unusual in that is has no valley.
The Niagara River was not formed
through the typical geological process whereby a river erodes a valley within
the landform. Rather, the Niagara River
was formed almost catastrophically in a short period of geologic time by very
large volumes of water flowing over the flat plain of the Niagara
escarpment and falling from its edge.
The erosive force of this falling water, and the characteristics of the
stone that was eroded, have determined the basic character of the aquatic and
terrestrial habitats of the gorge. Water
falling over the edge of the hard dolostone cap rock of the escarpment has
undercut and eroded the escarpment, forming seven miles of gorge in less than
15,000 years. The erosion of the gorge
has not been a steady, uniform process.
It has changed over time, with five different stages of gorge formation
recognized (Tiplin 1988).
The most important forces shaping the formation of the
gorge are the volume of falling water and the height of the falls. Lakes formed at the margins of the retreating
Pleistocene glaciers, roughly in the locations of the current Great
Lakes. Known as pro-glacial
lakes, they were very dynamic in terms of their size, shape, drainage basins,
elevations, and outlet locations. They
were sometimes much larger than the Great Lakes
and at other times, much smaller. The
total volume of flow through the Niagara region was initially much larger than
present, however, there were several outlets over the escarpment and only a
portion of the water flowed at the current location of the Niagara
River. Even after the
location of the gorge was fixed at the Niagara River, the drainage patterns of
the Great Lakes were still in a state of
flux. There were two periods when a much
smaller discharge from Lake Erie waters alone
cut the gorge.
Changes in the height of the falls have occurred as a
result of changes in the elevation of the receiving waters. For thousands of years, the outlet of Lake Iroquois,
the precursor to Lake Ontario, was through the Mohawk River valley and on
to the Hudson River. As a result, Lake Iroquois
was much higher and the shoreline abutted the north edge of the
escarpment. The first three stages of
the gorge were formed when glacial Lake
Iroquois was up to 125 feet higher
than the present elevation of Lake
Ontario. The higher elevation of Lake Iroquois
inundated the base of the falls. Not
only did this reduce the erosive force of the falling water, it also protected
lower rock strata from erosion during the first three stages of gorge
formation.
During Stage I of the formation of the Niagara Gorge, the
reduced fall of water stopped the erosion at a higher stratum of the
escarpment. Ancient terraces from the
location of the base of the falls are still visible in Lewiston (e.g., Eldridge Terrace). This was the first stage of the formation of
the Niagara Gorge, known as the “Lewiston Branch Gorge” or “Lewiston Spillway”
(Tiplin 1988).
It was created by a falls with a much smaller vertical drop and a much
lower flow since the Lake Erie precursor had
four outlets over the edge of the escarpment during this period (Tiplin 1988).
The Stage I gorge is relatively short, extending from the edge of the
escarpment upstream less than halfway to the RMNPP tailrace (Figure
2.0-1).
Stage II of the Niagara Gorge formation is called the “Old
Narrow Gorge” or “Erie Gorge” (Tiplin 1988). This reach extends form the end of the Stage
I gorge to a point just upstream of the current location of the RMNPP tailrace
(Figure 2.0-1).
During the second stage of gorge formation, the height of the falls was
still much reduced as a result of the higher water level of Lake Iroquois. More importantly, the discharge of the river
was much reduced since a large pro-glacial lake at the margin of the retreating
glaciers had found an outlet further east through the Trent
Valley in Ontario.
Most of the flow from the retreating glaciers passed through this
channel in the Trent Valley – the river flowing through the gorge only
drained the relatively small basin
of Lake Erie that existed
at that time. The erosive force of this
smaller river cut a much narrower and shallower channel. The smaller flow of the river also required a
long time to erode the Stage II portion of the gorge, much longer than other
sections of comparable length. This
section of the gorge was further modified by erosion when the lake level
dropped. This subsequent channel erosion
was carried out by the flowing waters of the river, as opposed to falling
water, since the falls had cut its way further upstream. This erosion also impacted the gorge walls –
both the Stage I and Stage II portions of the gorge are choked with wide talus
slopes of eroded and weathered rock, particularly the softer shales that are
exposed in these reaches.
The continental land mass under the glaciers was depressed
by the weight of ice. The land mass
rebounded as the glaciers melted and as a result, the Trent Valley
outlet of the pro-glacial lakes was eventually abandoned. This change in the outlet flow pattern began
Stage III of the Niagara Gorge formation.
During this period, the orientation and flow patterns of the Great Lakes were similar to the current conditions. The full flow of the melting glaciers passed
through the gorge. However, Lake Iroquois
was still at a high elevation since the outlet was still via the Mohawk and
Hudson valleys. The St. Lawrence valley
was blocked by glaciers during the third stage of gorge formation, preventing
flow through this potential outlet. Lake Iroquois
reached its highest elevation during Stage III – estimated to be as much as 125
feet higher than present Lake
Ontario (Tiplin 1988).
Thus, the fall of water was much less, although the volume of water was
large. During this stage, the falls
spread out over a wide arc of the escarpment and cut a wide channel, similar to
the current configuration of Niagara
Falls, but with a much smaller vertical drop. Stage III created the area known as Niagara
Glen on the Canadian side of the gorge.
On the U.S.
side of the gorge, all that remains of the large terraces below the falls are
the numerous small terraces perched along the gorge wall. The upper end of the Stage III gorge is
marked by several features; a pronounced widening of the channel, the upstream
margin of the Niagara Glen, and a river feature on the Canadian shore known as
Cripps Eddy. Subsequent lowering of Lake Iroquois
resulted in erosion by the river channel through the Stage III section of the
gorge. This later period of erosion
created a long stretch of rapids, known variously as Foster Rapids, Bloody Run,
or Devils Hole (Figure 2.0-1).
Stage IV of gorge formation includes several striking
changes that altered the character of the gorge. The level of Lake Iroquois
dropped, either due to rebound of the land mass or deepening of the outlet
channel in the Mohawk valley (Tiplin 1988). Lake
Ontario became much smaller when the
outlet eventually moved to the lower elevation of the St. Lawrence River – the
shoreline at the mouth of the Niagara River
may have been as much as 12 miles north of the current location. It was during this period that the riverine
erosion noted earlier further excavated the downstream portions of the gorge
(Stages I through III). At least two
small falls were created downstream when the river encountered the softer
sandstone and shale strata (Tiplin 1988). The lower water level increased the height of
the falls, which excavated a larger channel up to the location of the
Whirlpool. At the Whirlpool, the river
intercepted the buried St. David Gorge (Figure 2.0-1). The St. David Gorge was excavated during the
previous interglacial period and then buried by glacial debris and sedimentation. Since the material that filled the St. David
Gorge was unconsolidated, the falls moved rapidly upstream through this
material until it reached the location where the falls of the St. David Gorge
had previously existed. Intercepting the
St. David Gorge turned the river ninety degrees to the southeast and rapidly
excavated areas now known as the Whirlpool and the Eddy Basin,
two features that were originally formed by the St. David Gorge. These two deep basin features are separated
by a lip of harder sandstone. The
upstream end of the Eddy Basin was most likely where the old falls was
encountered, although Tiplin (1988) offers the
opinion that the older falls of the St. David Gorge may have been located
further upstream, near the Whirlpool
Bridge. The final event of the fourth stage of the
formation of the gorge was another reduction in flow. The reduced flow slowed the rate of erosion,
creating a narrower gorge through the Whirlpool Rapids. Alternatively, the Whirlpool Rapids were
formed by low flows in the St. David Gorge and then excavated during Stage IV
gorge formation.
The fifth, and final, stage of gorge formation resulted
from what are essentially contemporary conditions. The continued melting of the glaciers
completely opened the St. Lawrence valley outlet from Lake Ontario. Uplift of the land mass shut off the
alternate outlet from the upper Great Lakes. Thus, the entire discharge of the Great Lakes once again flowed through the gorge. This large volume of falling water, and the
great height of the Niagara Falls, excavated the
widest and deepest part of the gorge – the Maid of the Mist Pool, which extends
from the Whirlpool
Bridge up to the present
location of the Falls (Figure 2.0-1).
Figure 2.0-1
1913 Geological Map of the Niagara Gorge
3.0 NIAGARA RIVER HYDROLOGY
The hydrology of the Niagara River between the intakes and
tailrace is controlled by natural features of the channel (primarily cross
sectional area and gradient), and the diversion of water for the U.S. and
Canadian hydroelectric projects. The
natural features of the channel are described in Section 5.0, as they relate to
particular reaches delineated as part of this study. This section describes the regulatory
restrictions on water levels and flows, the operation of the hydroelectric
projects, and presents data on flows and water levels in the study area. All elevations in this report are referenced
to U.S. Lake Survey Datum 1935 (USLSD).
Values for other data, such as International Great Lakes Datum 1985
(IGLD 1985), are identified when used.
There are two regulatory constraints on flow and water
level fluctuations; the Treaty Between
Canada and the United States of America Concerning the Diversion of the Niagara
River, Oct. 10 1950, 1 U.S.T. 694 (Treaty), and the 1993 Directive of the International Niagara Board of Control
(Directive). Article IV of the Treaty
establishes a seven-month tourist season from April through October, and a
five-month non-tourist season from November through March. The Treaty requires a minimum flow of at
least 100,000 cfs over the falls during tourist hours from 8:00 AM to 10:00 PM
EDST during the period April 1 to September 15, and 8:00 AM to 8:00 PM from
September 16 to October 31. At all other
times during the tourist season, as well as during the entire non-tourist
season, the minimum flow requirement is 50,000 cfs. The Directive requires that the International
Niagara Control Structure be operated within certain water level restrictions
that are monitored at the Material Dock Gauge in the Chippawa-Grass Island
Pool. Additional information on the
Treaty, the Directive, and water levels, are contained in URS
et al. (2005).
Under Article V of the Treaty, Niagara
River water flows in excess of flows used and necessary for
domestic, sanitary and navigation under Article III of the Treaty, as well as
mandated flows over the Falls under Article IV, may be
diverted for hydroelectric generation purposes.
Article VI provides further that waters made available for power
purposes must be shared equally between the U.S.
and Canada. Water for the U.S. and Canadian hydroelectric
projects is withdrawn from the Chippawa-Grass Island Pool. Together, the hydraulic control provided by
the International Niagara Control Structure, and the withdrawal of water for
hydroelectric operations, determine the flow over Niagara Falls. When Chippawa-Grass Island Pool levels are
normal, NYPA has the capacity withdraws up to 102,000 cfs of water from the
upper river using two underground conduits that transport water to the Project
forebay where it is used for generation in either the Robert Moses Niagara
Power Plant, or the Lewiston Pump Generating Plant. Ontario Power Generation (OPG) withdraws up
to 65,000 cfs of water from an intake structure located close to the
International Niagara Control Structure.
OPG uses this water at the Sir Adam Beck Generating Stations located
across the river from the RMNPP tailrace in