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Runoff Report 1999.....Shuswap Lake

The 1999 runoff carried a high potential for flooding in the Shuswap Lake area. Clearly above normal snow levels was indicating excessive water supply throughout the runoff season. The low altitude snowpack was high enough to push the lake level up to advanced markings right in the beginning, while the medium and high altitude snowpack had the potential to bring a constantly large amount of water into the system during the main runoff. High temperature for a period of time or excessive rain periods could easily have led to a disaster.

Nevertheless, the 1999 runoff past with relative few problems. Minor flooding occurred on certain sections of streams and along the shoreline. The Eagle River near Sicamous reached 6 times critical levels where water spilled over into surrounding fields. Those spillovers happened between May 27th and July 13th before running out of excessive supply. I have to come back to that in the next section. Other streams may have had similar spillovers. Precise data is not available because of the lack of cooperation from local officials in this matter. But I am not aware of any major problems along other streams. The relative high lake level caused some problems for beach properties. Own observations from the Sicamous area showed some flooding of campgrounds as well as for some beachfront properties. Most property owners seemed to be good prepared and were able to hold most of the water back to avoid major damages. Overall, the damages during the 1999 runoff seemed to be relatively low.

It could have been much worse. The potential for disaster was always present, but was never activated. The reasons for that can be identified as:
  • low initial lake level
  • no excessive hot periods
  •      - The Time Span
  • mild winter
  • mild summer
  • runoff was divided into 5 major stages
  • no excessive rain in a short period of time
  • low nighttime temperatures
  • soil in low and medium altitude remained unfrozen
  • other lakes, able to buffer water

Some other factors may have been contributed to the total runoff but can be ignored based on their insignificant effect.

Before I get more detailed into the contributing factors let me start with some general observations and facts of the 1999 runoff.

The 1999 runoff first showed its effect on the Shuswap Lake on March 22, 1999, at a level of 344.91m ASL (= above sea level). In the following 99 days the lake level constantly increased until it reached its first peak on June 29, indicating a level of 349.199m ASL. The next 8 days the lake decreased to a level of 348.996m ASL before raising again for 7 days to its final peak on July 15, reaching its highest level of 349.227m after 115 days for the 1999 runoff.
After July 15 the lake level constantly and slowly decreased until it reached its normal in mid November.

The maximum daily level increase was noted on June 26 with a value of 16cm. This calculates to 0.7cm per hour. On the other hand, the largest daily level decrease was noted on August 2 with -7.2cm, which can be converted to -0.3cm per hour.

Most of the water brought into the Shuswap Lake is coming from 5 major streams:

  • Shuswap River
  • Adams River
  • Eagle River
  • Seymour River
  • Salmon River

Based on the total mass of inflow other contributions are rather insignificant and can be ignored. There are also no data available to include into the calculations. Those creeks and streams are mainly located close and with direct flow into the Shuswap Lake. Most carry the snow water very early in the runoff freeing the closer lake area from the snowpack. They seem to be back to normal flow long before the main runoff from the backcountry with minimal or no effect on the lake level. As for the larger streams please check the next chapter for further details.

First let's take a look at the 1999 lake levels. The graph is based on my own measurement during the period from April 15 to October 31. The blue line represents the actual lake surface level while the red line shows the daily level changes.

For the moment I will focus on the daily lake level changes. Following the red line clearly identifies 3 major phases inside the 5-stage total runoff (as explained later). Those phases are directly related to the remaining snowpack and the overall weather conditions:

Phase Period Snowpack Description
Start March 22 to May 11 Low Elevation Creeks and streams close to the Shuswap Lake carry the snow water from the low altitude snowpack into the lake. Larger creeks getting close their peaks (for example: Chase Creek on May 25). The major streams also bring water from the distance low altitude snowpack, reaching their first minor peak of the spring.
Main Runoff May 12 to July 19 Medium and Upper Altitude In general warmer weather leads to the melting of the snowpack in medium and higher elevations. The lake and all larger streams reach their peak for the runoff.
End July 20 to Mid of November High Altitude The high altitude snowpack continues to melt. The remaining snow-covered area is relatively small, therefore producing less water over the day. A usual longer distance from the snow source to the stream is slowing down the water flow, giving the ground a larger area to absorb the water and to release it slowly back into the stream network. Temperatures at night are often low enough to interrupt the melting process temporarily. With decreasing stream levels the lake starts to fall also.

The next chart best explains the effect of the weather to each of the just described phases.

During the first and most of the second phase the lake reacted very fast to changes of daytime temperatures. High temperatures always are followed by a clear increase of the lake level. By the end of the second phase this pattern began to change. The reaction of the lake was far less intense and the delay increased noticeable. This is a clear indication for the reduction of the remaining snow supply, which then was limited to higher altitudes, and the longer way the water had to travel from the source to the stream and lake.

The snow water takes a certain amount of time from the source to the lake. I call it lake reaction delay. This delay is depending on factors like snowpack, altitude, distance from the lake, temperature changes, soil conditions, backwards temperature patterns and intensity. Other factors like consitency of the snowpack and topographic conditions are disregarded here. With aid of the previous chart some temperature peaks were extracted and set in relation to the corresponding lake level changes. The difference between the dates calculates the mean delay in days.

The following table calculates the lake reaction delay for some temperature peaks:

Phase Temperature Lake Level Delay Average
I 19/04/99 22/04/99 3 2.8
24/04/99 26/04/99 2
30/04/99 03/05/99 3
05/05/99 08/05/99 3
10/05/99 13/05/99 3
II 17/05/99 19/05/99 2 2.6
19/05/99 22/05/99 3
23/05/99 26/05/99 3
31/05/99 02/06/99 2
03/06/99 06/06/99 3
15/06/99 17/06/99 2
30/06/99 02/07/99 2
06/07/99 09/07/99 3
12/07/99 15/07/99 3
III 22/07/99 25/07/99 3 3.9
28/07/99 30/07/99 2
06/08/99 10/08/99 4
11/08/99 16/08/99 5
16/08/99 19/08/99 3
19/08/99 22/08/99 3
24/08/99 26/08/99 2
28/08/99 01/09/99 4
02/09/99 07/09/99 5
21/09/99 25/09/99 4
04/10/99 09/10/99 5
10/10/99 14/10/99 4
17/10/99 23/10/99 6
24/10/99 29/10/99 5

Based on those calculations the lake reacts to temperature increases with a delay in Phase 1 of 2.8 days, Phase 2 of 2.6 days, and Phase 3 of 3.9 days.
The overall mean delay for the complete runoff calculates to 3.2 days. Understanding this fact provides a very important tool for the short-term forecasting. It has proven very effective and precise during last year's predictions. And most importantly, it gives enough time for flood preparations after announcing flood warnings. The next chapter will analyze some rivers in this respect.

There is also a difference in lake level changes during day and night. It seems logical since temperatures during the day are usually much higher than at night, which results in a larger snowmelt at daytime followed by an increased water flow into the lake. The accessible data does not allow delay calculations separated by day and night with a certain minimum on precision. But my own measurements may give some ideas about the general tendency in this regard. The lake level changes, separated by day and night, are shown on the next graph.

Lake level increases are always larger at night than at day. During the first phase those differences are minimal, but more recognizable during the second phase. This effect is reversed during the third phase, where the lake level decreases more at night than during the day. This allows the simple conclusion: regardless of decrease or increase, level changes for the Shuswap Lake are always larger during the night.

The weather is the most important factor during the runoff. Weather means not just temperatures. Equally important for the snowmelt are the following weather and other components:

  • intensity of the sun (clear sky or cloudy)
  • rain / rain temperature
  • temperatures over a period of time
  • wind and air temperature
  • heat absorption and heat storage of the soil
  • heat reflection from surrounding large objects and hillsides
  • topographic shape and location (valleys, hilltops,...)
  • angle of the snow surface to the sun

and some other more.

The following three graphs demonstrate a combination of

  • daily high / low temperatures
  • lake level changes, separated by day / night
  • some weather factors

A larger scale also will allow a more detailed view of the data. Please click at the links below to view the graph.

Part 1: April to May 1999

Part 2: June to July 1999

Part 3: August to September 1999

A continuum into later of the year is irrelevant. Please keep in mind that all the weather conditions are recorded in the Sicamous area. Given by the large area covered by the runoff the weather conditions could have been different at other locations. A regional separation would have required a more detailed data access, which I failed to gain. Nevertheless, the weather recordings provide a good indication for the overall conditions.

The last 3 graphs cover a timeframe of 166 days. The weather codes on the bottom represent for this period a total of

  • 66 days of rain or showers
  • 39 days with cloud cover
  • 61 sunny days with clear sky.

The 61 sunny days dictated the 1999 runoff. Rain had no or almost no effect on the runoff as clearly indicated on the graphs. This may be the result of mostly light rain or short rain showers without the potential of creating excessive water in a short time. The longest rain period happened in the second half of June and lasted 16 days. During this time the daily lake level changes constantly decreased, even causing the lake level to fall temporarily. Relative cool temperatures during this period may have aided to this fact by slowing down the snow melting process.

As described earlier, a number of factors had significant importance for last year's runoff. The next sections will give a short description to each factor.

At this point it will be assumed that you have read previous chapters of this report.

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Copyright (C) 2003 Bernhard Kramer, Sicamous, BC - Canada