Impounding a river to form a reservoir: e.g. the Oldman River Reservoir.
The most fundamental physical distinction among aquatic ecosystems is the distinction between flowing
water systems (rivers, streams) and standing water systems (lakes, ponds), and the most important
biological consequence of this difference is the formation of plankton (a rich community of tiny
organisms suspended in the water mass) in standing water systems. In most rivers, plankton is absent,
or at least not very abundant, because of physical turbulence. When a river is impounded by a dam, the
reservoir that is formed upstream of the impoundment will acquire many of the attributes of a lake.
The Oldman River Reservoir was constructed in 1989-91 downstream of the confluence of the three
main tributaries of the Oldman River, the Castle River joins from the south, the Crowsnest River enters
from the west draining the Crowsnest pass region, and the Oldman River mainstem enters from the
North. Historically, the averge annual runoff value (r) for this watershed is 400 mm, ranging from as
high as 1000 mm in the high alpine regions to as low as 100 mm in the lower portions of the Watershed.
Study the watershed map of the Oldman river, and familiarize yourself with the major tributaries of the
Castle, Crowsnest and Oldman Rivers using the detailed map provided in the accompanying powerpoint
file (Oldman River Reservoir Watershed.ppt). A report on the Oldman Reservoir was published by
Alberta Environment and has been uploaded as a reference for you to read.
(1) Using the Measurement tools on Adobe Acrobat Professional. Estimate the Area of the Oldman
River watershed at the damsite from the topographical map provided on the pdf entitled
Watershed Map. pdf. Now estimate the average amount of water expected to flow past the
dam in m3/yr and in m3/s. The actual amount measured during the 1990’s decade was 1514 x
106 m3/yr. How close is your estimate to this value?
The height of the dam is 76 m, and the base of the dam is situated at an altitude of 1050 m above sea
level (asl). The reservoir is considered to be filled to the maximum at elevation 1120 m. We can
estimate the volume of the reservoir when full to different levels by making a graph with the depth
contour value on the x-axis and reservoir area at that contour on the y axis; this is called a hypsographic
curve. The area under the hypsographic curve has units of km2 times m, so if we estimate the area of a
small rectangle corresponding to 10m on the x axis and 1km2 on the y-axis, we can calibrate the area
graph against this small rectangle which represents 10,000,000 m3. The mean depth if the reservoir is
filled to capacity would then be the total volume divided by the area of the full reservoir.
(2) Construct a hypsographic curve of the reservoir bathymetry data that have been estimated from
the Reservoir bathymetry map (Reservoir bathymetry. ppt). Calculate its total volume and mean
depth when full to capacity.
Depth
Elevation
contour Area
m, asl
(m)
(km2)
1120
0
20.2
1110
10
14
1100
20
10.2
1090
30
7.1
1080
40
5.2
1070
50
2.8
1060
60
1.3
1050
70
The reservoir was gradually filled during 1991-92. In April 1991 its level was at 1060 m, by May 1991 it
was 1080 m, by June 1992 the level rose to 1090, by August 1992 1100 m, and by Sept 1992 1110 m. It
reached 1120 (full) in June 1993. Since then the level has fluctuated seasonally between 1110 and 1120
m.
(3) Estimate the volume and mean depth of the reservoir at each of these dates.
The development of reservoir communities and the importance of water residence time
The biggest difference between a standing water ecosystem (lake, pond), and a flowing water system
(lake, river) is that standing waters generally support a rich plankton community (small algae, protozoa,
rotifers and microcrustaceans), whereas flowing water systems are generally too turbulent to allow
plankton to grow properly. Plankton organisms tend to remain suspended in the water mass, because
the rate at which small particles sink in water is extremely slow. While planktonic organisms are able to
remain suspended due to small size, if they can swim at all, it is very slowly—i.e. much slower than the
water mass is moving. Thus they are usually unable to avoid being swept downstream with flowing
water leaving a reservoir or lake, and are at the mercy of the turbulent river water once they have left
the lake or reservoir. Lakes and ponds with rich populations of large zooplankton usually have very
stable water columns.
When a river is dammed and a reservoir begins to form behind it, plankton community does not develop
immediately since the volume of water impounded is small, and the water is passing through the
reservoir too quickly to allow numbers to build up in the water column. Phytoplankton are usually able
to divide every few days, so if the water spends even a week or two in the reservoir, the phytoplankton
populations can grow exponentially through several generations and build up a decent community of
primary producers. Small animals (microzooplankton e.g. rotifers and protozoans) require up to a week
to complete their life cycle. Therefore, before they would become abundant, the reservoir would need
to be large enough that water would be entrained for several weeks, allowing these populations to go
through several generations of growth. Larger zooplankton such as Daphnia or copepods usually require
nearly a month to complete their life cycles, even when the water is warm, and although they usually
outcompete smaller animals like rotifers and protozoa when they live in lakes, many reservoirs do not
entrain water long enough (4 or 5 months) for these larger zooplankton populations to build up.
Fish on the other hand, except for tiny newly hatched larvae or fry can usually manage to resist flow and
remain within the flow regime of their choice. Therefore fish are for the most part able to resist being
advected (swept) downstream over the dam if they wish to. However, there is little plankton in a newly
formed reservoir to support them. Benthic animals (associated with the bottom) require even more
time to develop in the new reservoir than plankton; in fact, since reservoir levels often fluctuate a lot
(drawdown), littoral zones, the best habitat for benthic animals frequently remain very unstable and
biologically impoverished. Thus the ability of reservoirs to support fish communities depends greatly on
the establishment of plankton communities because zooplankton will generally be their main food. A
reference relating to plankton community development in a Canadian Reservoir has been included for
you to read (Campbell et al. 1998 Can J. Fish. Aquat. Sci.).
Many U.S. reservoir managers introduce a fish called gizzard shad into newly formed reservoirs, because
these fish have many long, fine gill rakers and can filter tiny particles from the water, even
phytoplankton. Thus, as soon as there is any kind of plankton community beginning to form in the
reservoir these fish can begin grow. Although gizzard shad aren’t palatable to humans, and moreover
tend to be small, they are therefore not suitable either as a recreational and commercial fishery.
Fortunately however, they make great forage for large mouth bass, a highly prized recreational species.
In Canada, however, our reservoirs are too cold to support gizzard shad, or any other suitable
subsititute, and fishery potential of Canadian reservoirs thus takes more time to build up since it is not
until large zooplankton like Daphnia and copepods are present in the reservoir that our common pelagic
fish species such as lake whitefish or kokanee salmon (or other zooplantivores) can be supported in a
reservoir. Indeed, such species may not even colonize the new reservoir unless either they are stocked
or a suitable source in upstream lakes exists. For these reasons, (i.e. instability associated with short
water residence, and drawdown, together with uncertain colonization) fishery development in
reservoirs is often uncertain, and at very least, requires a significant amount of time to develop. This
has often proved to be a source of frustration to dam operators or reservoir managers, who hope to
have a productive reservoir fishery to compensate for lost river habitat.
Water residence time is a measure of the time that an average molecule of water spends within the
reservoir. A crude measure of this can be obtained by dividing the volume of water present by the rate
at which water is flowing out. While the units for water residence time can be any time unit, the most
common usage is years. So to obtain an estimate of water residence time in years, divide average
volume by annual average discharge. Lakes with rich plankton communities dominated by large
zooplankton usually have water residence times on the order of a year or more.
(4) Provide a graph showing how the reservoir’s residence time would have been expected to
change over time (use the same dates as in question 3).
(5) Write a brief paragraph explaining the likely implications of your residence time graph for
plankton and fishery development in this reservoir.
Suppose on average the annual discharge entering the lake fluctuates from year to year due to climate
variation but power and irrigation demands require the outflow to be kept at or above 1000 x 106 m3/yr.
This is not unusual, since most reservoirs are managed with the aim of buffering variability in the flow
regime so as to maintain relative constant outflow in the face of highly variable inflows.
Assume that in an average year the annual flow entering the lake is equal to the outflow (ignore
evaporation) but that the input varies in such a way that the 25% percentile is 800 106 m3/yr and the
75% is 1500 x 106 m3/yr.
(6) Plot these data on probability paper, and then estimate the lowest input level expected in a
decade, and over a 50 yr interval?
(7) Assuming that in a sequence of three years the reservoir is full, followed by a 1 in 5 year drought
(20%-ile input) year, followed by a 1 in 10 yr drought (10%-ile input) year. What would you
expect the reservoir volume to be after this sequence? Would it still be possible to maintain the
discharge at the mandated minimum of 1000 x 106 m3/yr. What would be the expected water
residence time following the 1 in 5 yr drought? How would you have expected this to have
affected the plankton community of the reservoir.
(8) If an average year with the reservoir full, was followed by a wetter than average year with an
80%-ile input of water, what would the expected water residence time be at the end of this
year? What is the expected effect of variability in the river’s flow regime on the composition
and productivity of plankton in the reservoir?
o
Probability (percentiles)