Mountain Water Supply Summary

Last fall, soil moisture was normal to above normal in southern and central mountain and foothill areas, except for the Waterton - St. Mary area which was below normal to normal. Winter precipitation that followed was below normal to normal in the mountains and foothills. The water supply situation for this year was looking fairly good. However much of the southern snowpack melted in January as a result of rainstorms which edged in from British Columbia, where flooding resulted. A warm spring led to earlier than normal melting of the central and remainder of the southern snowpacks. As a result, natural flow volumes recorded during the March-April period were above to much above normal in most mountain-fed rivers of the Bow, Red Deer and North Saskatchewan River basins. However, below to much below average natural runoff volumes were recorded in most areas of the Oldman and Milk River basins during this period, due to the prior depletion of the snowpack in January and below to much below normal March-April precipitation.

By the time spring snowmelt runoff peaks usually start, there was little snow left to melt. Remaining mountain snow accumulations as of May 1, 2005 were generally much below average in the Oldman River basin and the southern half of the Bow River basin, and ranged from below to much below average north of this. The March through May recorded natural runoff volume total was the lowest on record for Brazeau Reservoir inflow, second lowest for the North Saskatchewan River at Edmonton, and 10th lowest for the Red Deer River at Dickson Dam and at Red Deer. March through May volume totals were generally near the 15th lowest on record in most areas of the Oldman River basin.

The upside of early melt was that soil moisture remained very good in most areas, and needed summer rain would yield a good amount of runoff. Natural runoff volume forecasts for the March-September 2005 period ranged from much below average in the Milk River basin to below average to average in the Red Deer and North Saskatchewan River basins.

June brought far more than normal rainfall however, to all of the five major water supply forecast basins. Water Supply forecasts were discontinued for the rest of the year, as flooding became the prevalent issue.

Total natural runoff volumes for the year (March - September) were much above average in the Red Deer River basin (Table 1), most of the North Saskatchewan (Table 2) and Bow (Table 3) River basins, and the northern half of the Oldman River basin. The Dickson Dam site recorded the fourth highest volume in 91 years of record, while volumes in most of these other areas were among the 15 highest on record. Exceptions include the inflow to the Bighorn Reservoir, which was average to above average, and the Bow River at Banff, which was much below average. The June storms, for the most part, did not get into the back ranges of mountains where these two areas lie.

The Waterton - St. Mary area of the Oldman River basin (Table 4), and the Milk River basin (Table 5) were spared from the heaviest precipitation during June. As a result, and in conjunction with previous months being dry, natural runoff volumes for the year were below average in these areas. Volumes ranged from 17th to 28th lowest in up to 91 years of record. Volumes in the Milk River basin are approximately two-thirds of average. However, much above normal June, August and September precipitation has resulted in a dramatic improvement, since March-May natural runoff volumes were less than a quarter of average.

Five months of forecasts for the three forecast points in the Milk River basin were on average within 9.8% of the actual March - September natural runoff volume. The first four of these forecasts were closer, anticipating good runoff from spring and summer rains on wet soils. However, a lack of precipitation in the spring and resulting low runoff led us to lower the May forecasts significantly. The May 1 forecasts were therefore over 20% lower than the recorded March - September natural volume, due to much above normal summer rainfall.

Four months of forecasts for the three forecast points in the southern Oldman River basin (Waterton, Belly, St Mary Rivers) were on average within 9.2% of the actual March - September natural runoff volume. In the northern Oldman River basin, where much more rainfall was recorded during June, forecasted inflows to the Oldman Dam were 55.2% lower than the recorded March - September natural volume. As a result, forecasts for the Oldman River at Lethbridge were 43.0% lower than the recorded March - September natural volume. Forecasts for other flooded areas in the Bow, Red Deer and North Saskatchewan River basins would be similar in accuracy.

Recorded volume data is preliminary and is subject to change. Check our Forecaster's Comments throughout the month for updated information regarding runoff conditions.