St Mary and Maxwell Lakes Bulk Water Supply and Climate Change Studies

Project Highlights

The North Salt Spring Waterworks District (NSSWD) relies on St Mary and Maxwell Lakes to supply potable water for about 5,500 customers on Salt Spring Island.  The NSSWD has water licences to withdraw annually up to 1,202,470 m3 and 663,729 m3 from St Mary Lake and Maxwell Lake, respectively.  Recent dry summers have resulted in extremely low water levels in both lakes, raising concerns about whether available storage is sufficient to support water demands.  Compounding this concern is projected increased demand coupled with climate change forecasts of warmer summers and prolonged dry periods.

KWL was retained by NSSWD to complete hydrological studies to assess whether water yields at St Mary and Maxwell Lakes can support demands up to the licenced withdrawal limit.  To determine the water system’s vulnerability to projected climate change, a separate assessment, which reviewed potential future conditions to the 2050s, was carried out.  KWL used a monthly water balance model, a GIS-based hydrologic model developed in-house, to produce estimates of monthly water availability from both historical climate data and future climate, based on global climate change model projections.

The results of these studies indicate that both lakes can support current water demands up to a 10-year-return-period drought (occurring in about 10% of years).  Current storage at St Mary Lake, however, may be insufficient to support increased demand beyond the licenced withdrawal limit.  These studies have given the NSSWD the information it needs to move forward with its plan to increase storage at St Mary Lake to meet future water demands.