Recent rain and snow in the Dolores Basin have improved the water-supply forecast for the reservoir and lower river, says Mike Preston, manager for the Dolores Water Conservancy District.
“We’re back on track for a small, short spill,” he said.
Colorado’s second-largest reservoir is expected to fill and provide farmers with a full allocation.
While not guaranteed yet, the runoff forecast shows enough leftover for a five to 10 days of rafting below the dam at a rate of 1,000 cubic feet per second.
In the next week, reservoir managers will announce any planned boating releases, and will target the Memorial Day weekend. However, given the cool weather pattern it is possible that all or part of the spill will come after Memorial Day.
Ramp-up could begin late next week, and will be announced on the www.doloreswater.com website under the “releases” tab.
Whether there will be a spill has been a big question for the boating community in recent weeks.
For months, a small spill was deemed probable. On May 1, the forecast called for no spill, then this week a short one was predicted.
The reasons for the on-again, off-again predictions have to do with the variability of runoff forecasting.
Early winter snowpack brought February 1 to 150 percent of median for the Dolores Basin. Managers were confident that there would be a full reservoir for farmers based on historic averages February through May. A 100,000 acre-feet carryover in McPhee after the 2015 irrigation season was also key factor for filling the reservoir in 2016.
Boaters dared hope there would be too much runoff for the dam and reservoir to store, triggering a controlled release of boatable water flows. The last release was in 2011.
DWCD relies on the Colorado Basin Runoff Forecast Center for “most probable” predicted water supply from the Dolores River into McPhee Reservoir.
A March 1 prediction of a three-week rafting season in May and June fueled rafters’ expectations.
But a weak February and March put the spill in jeopardy. By April 1, snowpack in the Dolores Basin had dropped to 88 percent of the 30-year median.
Predictions for a spill waned. It went from three weeks to a mere five-day spill forecast on April 15. Then on May 1, no spill was forecast.
On May 16, the forecast center’s prediction improved, and a spill was again forecasted as possible in part because of rainy weather, remaining snowpack and low irrigation demand.
“We had some significant precipitation, resulting in an increase of 14 percent in projected inflow,” Preston said. “We have learned that May rains can dramatically reshape the prospects for a small spill because they increase flows and slow the onset of irrigation use.”
Managers explained that in mild to average winters predicting whether runoff will be enough to “fill and spill” becomes too close to call because of the inexact science of predicting the variability of snowmelt, runoff, precipitation and evaporation due to wind, dust and heat.
“2016 has reinforced our understanding that heavy snow is critical in December and January, because we can’t count on February forward to produce enough high snow,” Preston said.
To better refine runoff forecasts, DWCD added a 6th Snotel at Black Mesa in 2013. Another goal is improved modeling for the timing of how snowpack comes off in relation to elevation, slope aspect, and response to variations in temperature.
The main Lower Dolores River boating run stretches for 100 miles through winding, red-rock canyons interspersed with rapids ranging from Class I to Class IV. It is considered one of the premiere multi-day boat trips in the country when it has enough water to run. No permit is required.