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Natural Resources Conservation Service
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Boise, Idaho 83709
 
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Cold and Dry Weather Delays Snow Melt
May Water Supply Outlook Report for Idaho by Dan Gallagher for the NRCS

Boise, Idaho, May l 8, 2008 – Idaho’s water managers, farmers and recreational boaters have had their wishes granted with ample snow in many basins and cool temperatures that have kept it in the high country. Now, they hope that unseasonably warm temperatures this month do not send high volumes of snowmelt flowing down the rivers, especially in the northern half of the state. The U.S. Natural Resources Conservation Service has released its Water Supply Outlook Report as of May 1.

Idaho’s precipitation trend for the past few months was downward, and below normal in most basins. If that had occurred in the middle of the winter, it could have meant inadequate snow. But April temperatures were 6-10 degrees below normal in the Pacific Northwest and Northern Rockies.  That added more snowfall in higher elevations and preserved the snowpack accumulations into May. Early April is traditionally the time when Idaho’s snowpacks are their highest before melting and heading downstream.

Following a mild winter of 2006-2007 and the depletion of carryover in the reservoirs, Idaho faced the prospect of inadequate water for this summer if the snowstorms dodged the state.  April precipitation ranged from 90 percent of average in the Clearwater and Panhandle Region to 40 percent in the Big Wood, Little Wood and Big Lost basins. Across central Idaho, which includes the Salmon, Weiser, Payette, Boise, Wood, and Lost river basins, it was the third consecutive month of below normal precipitation.  But the cool weather meant snowpacks just reached their peak amounts at the end of April in the Panhandle, Clearwater, Salmon, Payette, Henry’s Fork, Teton and Upper Snake basins. The highest snowpacks remain along the west side of the state from the Weiser basin at 168 percent of average to the Kootenai River basin at 121 percent. The snowpack is 140-160 percent of average in the Coeur d'Alene, Spokane, North Fork Clearwater, Lochsa, and Selway basins.  To the south, the Salmon Basin overall snowpack is 120 percent. The Boise and Little Wood basins are about 115 percent of average; Big Wood basin is 106 percent. The snowpack decreases to near-normal levels in the Big Lost and Little Lost basins.

The Snake River basin snowpack above Palisades Reservoir is 116 percent of average. Basins across southern Idaho's high desert range from 120 percent of average in the Owyhee to 143 percent in the Raft River. The Bear River snowpack is near average.

The highest streamflow forecasts call for near 140-150 percent of average in the Clearwater basin and Panhandle Region. Moderate weather would allow high-elevation snow to melt slowly, while a hot spell would cause rapid flow increases and potential flooding, especially in northern Idaho. The lowest streamflow forecasts are 80-90 percent of average for the Wood and Lost river basins. Near-normal volumes are predicted in the Boise, Upper Snake, Salmon Falls, and Oakley basins. The Bear River is forecast for slightly above average flows in the headwaters and decreases to 43 percent of average for the Bear River at Stewart Dam.

Most reservoirs are 50-80 percent of capacity and are maintaining storage space for the snowmelt runoff. Whether or not reservoirs fill completely or not may come down to timing of runoff and irrigation demand. April inflows to Bear Lake, Oakley and Salmon Falls reservoirs have been minimal and the reservoirs are only are 20-40 percent full and will not fill. Surface water irrigation supplies should be adequate in most basins, but could be tight for the Bear, Big Wood, Little Lost, Big Lost and Salmon Falls water users.

The ample snow pack in central and northern Idaho bodes well for recreation. The delayed melt could put a damper on wildfires and the resulting smoke clouds, or delay them to later in the summer. The Middle Fork and South Fork Salmon River have ample snowpack, but floating logs and debris from the 2007 fires could be present early on. Paddlers on the rivers should use caution, especially as the river flows reach their peaks. The Main Salmon River is forecast at 103 percent of average and will be sediment ridden during the peak flows which could exceed 70,000 cubic feet per second.  The Payette River will have a long season as the reservoirs release irrigation water well after the seasonal peaks occur. It appears the Owyhee, Weiser and Camas streams have peaked from snowmelt, but additional rains could kick them in again. The Bruneau River, which is forecast at near average, will have a good floating season.  Here are some snapshots of Idaho’s basins on May 1:

Panhandle Region

The overall snowpack for northern Idaho hit 129 percent heading into May. The greatest amount is found in the Rathdrum Creek area with well over the twice the average amount, while the Coeur d’Alene and Spokane basins are more than 150 percent. The risk of unregulated streams overtopping their banks this spring is a concern, especially when May temperatures start to escalate.

Clearwater River Basin

Comparing the major river basins, the Clearwater has the highest snowpack this year ranging from 139 percent of average in the North Fork Clearwater, to 152 percent in the Selway and 155 percent in the Lochsa. Dworshak Reservoir is currently storing 72 percent of average, 50 percent of capacity and the May-July inflow is forecast at 122 percent.

Salmon River Basin

The Salmon basin snowpack ranges from a high of 160 percent of average in the Little Salmon to 107 percent of average in the Lemhi. The Middle Fork Salmon River generally peaks when the snow at Banner Summit SNOTEL is half melted. This year, Banner Summit peaked at 28.4 inches of snow water on April 12. Since then only a couple of inches have melted off.

Weiser, Payette, Boise Basins

The Weiser basin snowpack is 168 percent of average, highest May 1 since 1999. Overall, the Payette basin snowpack is more than twice last year but still less than in 2006. The entire Boise basin is 114 percent of average, twice last year but much less than in 2006. The Payette reservoir system is 90 percent of average, 59 percent full and is ready for the runoff as the ice comes off Lake Cascade. The Boise reservoir system is 95 percent of average, 65 percent full and should fill.

Wood and Lost River Basins

Currently snowpacks are slightly above average as a whole. Most streams are forecast at 80-90 percent of average for the May-July period. Tight water supplies are anticipated.

Upper Snake River Basin

Current snowpack percentages in most drainages are about 120 percent of average. A few exceptions are the lower-elevation Willow, Blackfoot and Portneuf drainages at 142-198 percent of average. Combined storage in Palisades Reservoir and Jackson Lake is 82 percent of average, 48 percent full; these will be the hardest reservoirs to fill. The system should fill or come close but final fill will depend on timing of runoff and irrigation demand in May and June.

Southside Snake River Basins

Snowpacks are near 140 percent of average in the Raft and Oakley basins, and 120-130 percent in the Salmon Falls, Bruneau, Reynolds and Owyhee basins. Water supplies should be adequate but could be tight for the Salmon Falls tract farmers, depending upon spring rains and irrigation demand.

Bear River Basin

Current storage in Bear Lake is 430,964 acre-feet, about 30 percent of capacity and 44 percent of average. The forecast indicates that surface water supplies should be marginally adequate for the Bear Lake water users.

The full NRCS May 1 Water Supply Forecast Report for is available at: http://www.id.nrcs.usda.gov/snow/watersupply/.  The report provides a summary of snowpack conditions, precipitation, reservoir storage, and forecasted streamflows at monitoring points in each major basin in the state.

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