March turned out to be a drier month than a normal Idaho March, according to Ron Abramovich, water supply specialist with the Idaho Natural Resources Conservation Service, but February’s major storms almost guarantee adequate water for users this year.
The gradual melt of snowpack in the lowest elevations last month helped relieve some of the pressure of a rapid melt, he said in his monthly water supply outlook. However, the mid-elevations across Idaho “still have a tremendous snowpack that is ripe and ready to melt.” That snow won’t melt until rain and warmer temperatures prompt melting.
Spring weather in the next few weeks will determine how this year’s snowpack melts and fills rivers and reservoirs, Abramovich wrote. March temperatures in Idaho and most of the West were near to slightly below normal. With longer days in April, the potential for warmer daytime temperatures exists, but as long as it continues to freeze at night, the melting process is slowed, he said. Consecutive days of rain saturates the soil and provides greater runoff than if precipitation falls every other day or every few weeks. Short-term weather outlooks call for more moisture for the next several weeks.
According to Abramovich, the pattern of storms bringing rain in the valleys and snow in the higher elevations is expected to continue in April. After more of the mid-elevation snow melts, the higher elevation snowmelt will keep rivers flowing near average or better into the later summer months.
March was a relatively calm month in the Salmon River basin, according to Abramovich’s report, in terms of weather. Cooler than normal to seasonally normal temperatures left the snowpack relatively undisturbed after copious snow in February.
The vast Salmon River basin snowpack is above normal at about 110 percent with lesser amounts to the east toward Montana and more in the central Idaho mountains that make up the Salmon River headwaters.
Monthly precipitation in March was the lowest so far this water year — 63 percent of average — bringing water year-to-date precipitation to a normal level of 101 percent in the Salmon River basin.
“February likely sealed the deal for a normal water supply in the Salmon River basin,” he wrote. A few wet months leading up to the heat of summer could cause rivers to run bigger than expected, while a few dry months could result in slightly less than normal runoff. Normal to slightly above normal runoff volumes are expected. Abramovich expects plentiful water supplies for irrigation, fish, rafts and boats, fishing and other recreational interests in the Salmon River.
Like the Salmon River basin, March snow accumulation in the Wood and Lost River basin was low. But February’s heavy snow means snowpack there is still above average, especially in the 5,500 to 6,500 feet elevation range. Snowpack conditions on April 1 ranged from 120 to 160 percent of normal in the Wood and Lost basin.
Mackay Reservoir is 74 percent of capacity, or 106 percent of average. Little Wood is 19 percent full, 28 percent of average and Magic Reservoir is 44 percent full, 94 percent of average. All three reservoirs are expected to easily fill from snow-driven runoff. Streamflow forecasts generally range from 130 to 180 percent of average.
“Water supplies will be more than adequate across the Wood and Lost River basins,” Abramovich said.
“Reservoir operators are watching inflows and streamflow forecasts to assist in their delicate balancing act of making releases while ensuring reservoirs refill after peak inflows occur,” Abramovich wrote.
He expects most reservoirs and natural lakes in Idaho to fill this year, which means carryover storage for next year. It’s unlikely that Bear Lake, Salmon Falls and Oakley reservoirs will fill, and it’s too soon to say if Owyhee Reservoir will fill, he said. The good news for reservoirs, according to Abramovich, is enough snow has already fallen in the mountains to ensure adequate irrigation supplies this season.
Streamflow forecasts mirror the snowpack and increase from northern Idaho to southern Idaho. Streamflow forecasts in the Salmon basin range from 99 percent of average for the Lemhi River to 109 percent of average for the Main Fork Salmon River. The Little Wood River is at 175 percent and the Big Lost River is at 132 percent.
Early season river runners should get ready, Abramovich said. For people who prefer to float on long summer days, he predicts good river flows later into the summer this year.
Precipitation in central Idaho last month was about 50 percent of normal, Abramovich said in his April 1 report. The Salmon basin stands at 109 percent of normal. The Little Wood basin is at 125 percent of its water year-to-date precipitation.
The mid-elevation snowpack in the 6,000 foot elevation band across central Idaho is well above normal. The Little Wood basin is at 160 percent of normal. Overall, most basins south of the Salmon basin range from 110 to 145 percent of median. The Little Lost and Salmon are on the low end, near 110 percent.