This report has been prepared for Tolko Industries Limited (Tolko), BC Timber Sales (BCTS) and the Westbank First Nation (WFN) to update the watershed ECAs, assess the potential hydrologic impacts of the loss of forest cover as a result of the expansion of the mountain pine beetle, provide input for the development of the retention plans and finally a review of the complete retention plans. The last watershed assessment or IWAP was completed for the watershed in 2001 that updated the original 1998 report. The hazard ratings were low for sub-basins except for the North Fork where the peak flow and the channel stability hazards were rated as moderate. This current assessment uses the same sub-basins as were used in the earlier reports. The loss of most or all of the remaining mature lodgepole pine in the Lambly Creek watershed will affect the hydrology. The mature lodgepole pine in this watershed was also attacked in the late 1970s resulting in an aggressive harvesting program at that time. It is the loss of canopy and related decrease in evapotranspiration combined with increased water yields, particularly in the snow accumulation zone in the upper watershed area that will impact the hydrology. The primary area of concern is the snow sensitive zone, the source for the spring freshet and peak stream flows. Loss of canopy closure results in increased snow accumulation on the ground and subsequent increase in melt rates. The decrease in evapotranspiration due to the tree mortality further exacerbates the problem resulting in the potential for even greater water yields. The impacts on the magnitude and the timing of peak flows are a concern in the areas above the reservoirs due to the location of the snow sensitive zone as well as in the southwest corner of the watershed that drains into the
mainstem above the intake.