Oldman River Basin - Water Supply Forecasts

As of May 1, 2005, natural runoff volumes for the March to September 2005 period are forecast to be below to much below average for Lethbridge, the Oldman Reservoir at Brocket and the Waterton River, and much below average for the St.Mary and Belly Rivers (Table 3).

Current forecasted values for the March to September 2005 period for the Oldman River at Lethbridge would rank 17th lowest in 91-years of record (1912-2001). Current natural runoff volume forecasts range from 66 to 76% of average. The March to September 2005 forecast volumes are 2% higher than those recorded during the same period last year for Brocket, and 3 to 16% lower than last year for Lethbridge and the Belly, Waterton and St. Mary Rivers. The May forecasts are 1 to 3% lower than last month's due to precipitation during April that was 50 to 80% of normal.

The first two months of the forecast period have been completed, with March-April 2005 recorded natural runoff volumes ranging from 54 to 90% of average. Runoff ranged from below average to average in the Waterton and Belly Rivers and at the Oldman Reservoir, and was below to much below average at Lethbridge and much below average for the St.Mary River. Recorded volume data is preliminary, and is subject to change. Forecasts for May through September are for below to much below average natural runoff volumes. This remaining volume forecast, which is the March to September 2005 forecast minus the recorded volume to date, is available in Table 3b.

Although snowpack in the area was severely depleted in January by rainfall and warm temperatures, future precipitation can still heavily influence the forecast runoff. If minimal precipitation occurs during the coming months, near record low volumes are possible. Conversely, above normal precipitation could yet result in near average runoff volumes.

Also, reservoirs were able to store some of the early runoff volume in January, and as a result most are currently at above average levels.