Natural Factors Affecting the
Level of Osoyoos Lake
Background
In
a natural lake (i.e. one with no artificial controls such as a dam at
its outlet), the level of the lake is generally controlled by the inflow
and the geometry of the outlet. The
lake level determines the outflow.
In a natural system, a lake will always adjust its level in an
attempt to balance inflow and outflow. When the inflow exceeds the outflow,
the level of the lake will rise to provide the additional energy, or
head, necessary to increase the outflow. Conversely, if the inflow is less than the
outflow, the lake level will drop and outflow will decrease. If inflow remains constant, the lake will find
the level required to produce outflow equal to inflow.
When
there is a dam at the lake outlet, such as Zosel Dam for Osoyoos Lake,
outflow can be controlled by opening or closing gates in the dam. Obviously, if it is controlled such that outflow is less than inflow,
the lake will rise, and vice-versa.
In
the case of Osoyoos Lake there is another complication.
The Similkameen River, with a mean annual flow 3.5 times greater
than that of the Okanogan River, joins the Okanogan just below Zosel
Dam. Because the land at the confluence is flat,
high water levels in the Similkameen River effectively impede or block
the flow out of the Okanogan River and Osoyoos Lake.
With extreme high water in the Similkameen River, flow in the
Okanogan River may reverse and flow upstream into Osoyoos Lake. This
is a natural phenomenon that would occur whether there was a dam at
the outlet of Osoyoos Lake or not.
Control of Osoyoos Lake Levels during non-drought
years
The
following is a summary of the various conditions that can occur in Osoyoos
Lake during the period April through October in a year in which runoff
is average or greater.
1. Normal inflow to the lake, normal flow in the
Similkameen River
Osoyoos Lake inflows less than
2,000 cubic feet per second ( ft3/s) (57 m3/s),
and Similkameen River flow less than 10,000 ft3/s (283 m3/s).
Under these conditions there is no outflow restriction downstream from
Zosel Dam due to the Similkameen River and the gates are operated to
maintain the level of Osoyoos Lake between 911.0 and 911.5 feet as required
by the IJC. There is relatively
little difference between the water level at the lake and at the dam
(Figure 2).
2. High inflow to the lake, normal
flow in the Similkameen River
Osoyoos Lake inflows greater
than 2,000 cfs (57 m3/s), and Similkameen River flow less
than 10,000 ft3/s (283 m3/s).
Although there is no restriction downstream from Zosel Dam from the Similkameen
River, high inflow may cause the level of Osoyoos Lake to exceed 911.5
feet. Under the terms of the IJC Order of Approval, the gates at Zosel
Dam are to be fully opened whenever the level of Osoyoos Lake exceeds
911.5 feet, unless a drought has been declared. During high inflows,
the level of Osoyoos Lake will rise to provide enough head to force
water out of the lake. For example, at an inflow of about 3,000 ft3/s
(85 m3/s), the lake would have to rise to about 913 feet
for the outflow to balance the inflow.
Inflows are unlikely to exceed 3,700 ft3/s
(105 m3/s) which would be associated with a lake level
of about 913.5 feet (Figure 3). However,
it should be noted that lake levels greater than 913 feet are likely
to occur more frequently due to the conditions outlined in the following
paragraph.
3. Outflow Restricted by Similkameen River flows
Flows in the Similkameen greater
than 10,000 ft3/s (283 m3/s).
The Similkameen River is a natural river with no controls. Its peak flows can be more than ten times as
great as those in the Okanogan River leaving Zosel Dam.
The relationship is complex, but, in general, when the flow of the Similkameen
River as measured at Nighthawk exceeds about 10,000 ft3/s
(283 m3/s), the Similkameen River level becomes high enough
to restrict outflow in the Okanogan River.
The greater the flow in the Similkameen, the greater is this
blocking effect. (Figure 4). If the outflow is restricted to less than the Osoyoos Lake inflow,
the lake level will rise. Under
certain circumstances it is possible for the level of the Similkameen
River to exceed the water levels at Zosel Dam with the result that the
water in the Okanogan River flows upstream into the lake (Figure
5). This reverse flow only occurs relatively rarely,
the last occurrence being in 1976.
What can be done?
High Osoyoos Lake levels (other than during a drought) are a natural phenomenon caused by hydrologic factors and the natural physical layout of the area. As a result, little can be done to reduce Osoyoos Lake levels when high flows occur. Two commonly suggested remedies are:
1. Reduce flows into Osoyoos Lake
When high Similkameen River flows occur, every effort is made to reduce
Okanagan River flows into Osoyoos Lake to reduce both the rate of rise
and peak lake levels. However,
flows greater than 10,000 ft3/s (283 m3/s) in
the Similkameen can be continuous for several weeks in high runoff years
- which also tend to be high runoff years in the Okanagan basin in Canada.
This makes it impractical to cut flows in the Okanagan River
for an extended period.
2. Lower the level of Osoyoos
Lake in anticipation of high flow in the Similkameen River
Osoyoos Lake cannot ameliorate high flood levels because the storage volume
available in the lake is very small compared with the total flow volume
into the lake. At low Osoyoos
Lake levels, the outflow capacity is also low.
The lake level can rise by several inches per day if there is
a large difference between inflow and outflow.
Thus, any benefit of lowering the lake level in advance would
be lost in only a few days once inflows increased and/or outflows were
restricted. When there is a large difference between inflow
and outflow, the lake would rise rapidly and there would be little or
no difference in the peak level attained by the lake nor the duration
of high lake levels. Consequently,
attempts to hold Osoyoos Lake at a lower level in anticipation of high
runoff are of little value.
