Runoff Report Lake


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Runoff Report 1999.....Initial Lake Level

The constant inflow and outflow of water regulates the lake with the result of ever changing levels. The volume of the outflow depends only on the lake level itself (disregarding factors like ice cover, etc.). The next graph illustrates the relation between the lake level and the lakes outflow, based on the actual 1999 data between April 01 and October 31.

During and after a warm and dry summer with low snow cover in the mountains the lake inflow is much lower than the outflow. This results in an increased draining of the lake during the winter. By the beginning of the following runoff the lake level is below its normal height. This creates an increased buffer effect. During the first runoff stage the lake slowly reaches its normal level while already draining a large portion of the snow water. Therefore the main runoff shows its effect on the lake later as usual in the year and not as extreme.

The importance of the initial lake level can be best explained by comparing the runoff year of 1999 to 1997 (which caused some flooding in this area) and 1972 (last massive flooding). To have comparable data I selected the date of April 08 as start level. Take a look at the table below.

(all values in m ASL)	1999	1997	1972
Lake Level on April 08	345.25	345.75	345.75
Peak Level	349.227	349.32	349.66
Lake Level Increase	3.977	3.57	3.91

Source: District of Salmon Arm (Lake Gauge Recordings)

The start level on April 08, 1999, was 0.5m below of that in 1997 and 1972. But the lake level increase was the highest, even higher than in 1972 (!). As I will analyze later the weather conditions couldn't have been more cooperating in 1999, resulting in an unusual slow and late main runoff. It doesn't need much explanation what could have happened if the start level in 1999 would have been higher. The lake probably would have reached or even exceeded the 1972 peak level resulting in massive flooding with serious effects reaching as far as Vancouver. And adding to that a normal summer for 1999 with more stable temperatures, this area easily could have seen its worst flood disaster in BC's history. Most people don't realize how lucky we were.

The reasons for the low lake level in early 1999 are:

relatively low snowpack during the winter of 1997/1998
low and early peak level of the lake in 1998
a hot and dry summer of 1998 with decreased lake inflow

Some of that is explained further below.
In 2000 we don't have this kind of buffer capabilities in the lake.

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