1 Extent of Snow Cover During the 2002 Spring Freshet For the TROUT CREEK WATERSHED (Second Year of Snowline Data) (Penticton Forest District) 1.0 INTRODUCTION The extent of snow cover over a watershed basin has long been recognized as a prime variable in the determination of snowmelt generated peak flows. Direct observations of snow cover are an important tool in forecasting both the volume and the potential magnitude of peak flows generated during the spring freshet period 1. In the spring of 2001, and 2002 aerial reconnaissance was carried out over the Trout Creek watershed to determine the extent of snow cover. The flights occurred on May 2 and 11 of 2001, and May 13 and 27 of During the overview flights, oblique photographs were obtained and the location of the snowline was mapped. 2.0 OBJECTIVES The snow cover survey program was initiated in 2001, and continued in 2002, to aid in the evaluation of the potential for various forest development planning scenarios to alter spring freshet peak flows. The data have been utilized as a first step in guiding the evaluation of various proposed forest development planning scenarios. The 2001 and 2002 snow cover information has been used to determine the area in the Trout Creek watershed that contributes the largest amount of water to peak flows in Trout Creek. That is, areas that are snow covered near the date when peak flows occur on Trout Creek are more sensitive to forest development in terms of increasing peak flows. Future surveys of snow cover should assist in improving analyses of possible forest development planning scenarios and could be used to adapt forest development planning to minimize any potential hydrologic impacts. 1 USACE (U.S. Army Corps of Engineers) Snow Hydrology, Summary Report of the Snow Investigations. North Pacific Division, U.S. Army Corps of Engineers, Portland.
2 Trout Creek Snowline Survey: Spring 2002 Page RESULTS OF 2002 AERIAL RECONNAISSANCE Flow data for Trout Creek were unavailable for analyses in this report because Environment Canada terminated the operation of the Trout Creek hydrometric stations in However, Environment Canada continues to collect flow data on Camp Creek (a tributary to Trout Creek) near the confluence with Trout Creek. The Camp Creek flow data were used as a surrogate for Trout Creek flow data in terms of estimating the timing of peak flows. It is likely that minor inaccuracies resulted from this substitution since Camp Creek is much smaller than Trout Creek. However, the highest elevation in the Camp Creek watershed (~1900m) is similar to that for the entire Trout Creek watershed (~1900m), which suggests that the snowpack persists as long in the Camp Creek watershed as in the entire Trout Creek watershed. As long as the snowpack conditions are similar between the two basins, the timing of the peak flows should be similar. In addition, the nearest snowpack data (snow water equivalent; SWE) available are from the snow pillow at Brenda Mine and Mission Ridge. These data were used to represent the possible range in snowpack conditions in the Trout Creek watershed. This appears reasonable since Brenda Mine is located in the adjacent Peachland Creek watershed and the snow pillow is located at an elevation of 1460m (similar, but slightly lower to the snowline elevation in the Trout Creek watershed during the peak flow period). As well, the Mission Ridge snow pillow is located at an elevation of 1850m (near the upper most elevations of Trout Creek) in a similar sized watershed. There were two spikes in the discharge in Camp Creek at the mouth in 2002, one on May 2, the other on May 29 (refer to Figure 1). The May 2 spike was 0.795m 3 /s and the May 29 spike was and 1.419m 3 /s. The runoff peaked on May 22 with a maximum discharge of 1.63m 3 /s. In the spring of 2002, aerial reconnaissance was carried out over the Trout Creek watershed using a fixed wing aircraft to determine the extent of snow cover. On May 13, 2002 the snowline in the Trout Creek watershed was mapped at an elevation of between 1350m to 1450m. On May 27, 2002 the snowline was mapped at an elevation of between 1500m to 1600m. It should be noted that the snowline elevations identified are not absolutes but a range at which the snowline occurs, depending on such variables as slope and aspect.
3 Trout Creek Snowline Survey: Spring 2002 Page CONCLUSIONS The annual maximum daily discharge of 1.63m 3 /s for the spring freshet period occurred on May 22. This peak discharge appears to be related to snowmelt as well as rainfall. In terms of selecting a snowline that corresponds to the start of the period of peak flow that can cause channel changes, neither of the two mapped snowlines (May 13, and 27) are adequate. The May 27 snowline survey occurred after the annual peak flow. The May 13 snowline survey occurred too early in the peak flow period at a time when flows were relatively low compared to the peak flow (refer to hydrograph in Figure 1). As a result, it was decided to interpolate a snowline using the May 13 and May 27 snowlines. Using the data from the Mission and Brenda Mines snow pillow and the flow data for Camp Creek as a guide, it was estimated that the start of the rapid melt period leading to the peak occurred on approximately May 18 (refer to Figure 1). Based on this information, a snowline was interpolated, which varies between 1400 and 1500m in elevation. Based on the 2002 data, the watershed area that is situated above the interpolated snowline is most sensitive to forest development with regards to increasing peak flows. The following table summarizes the results for each year of the snowline study. Table 1 - Results of Annual Snowline Survey Year Date of the Start of the 2002 peak flow period Snowline Elev. at start of Peak Flow period 2001 May May With only two years data, the data set is not sufficient to definitively identify the lower limit of the zone most sensitive to forest development with regards to increased peak flows.
4 Trout Creek Snowline Survey: Spring 2002 Page RECOMMENDATIONS The snow survey program should be conducted through 2003 to 2005 to complete the minimum five year recommendation so sufficient data can be compiled to isolate the lower limit of the snow sensitive zone. It is recommended that four or five flights should be conducted each year to ensure that the lower limit of the snowline is captured during the peak flow period. Based on the future available budget for the snowline survey project it is recommended that a threshold for stream flow be determined and applied to all years of data to further isolate the lower limit of the snow sensitive zone.