Observed April-July volume flows for rivers in the San Juan Basin were much above average compared to much below average last year. Based on a sample population of 11 stream gages, spring runoff flows (April -July) ranged from 120%-170% of normal with a median of 135%. Streamflow was much above average during March due to warm temperatures and early snowmelt.
Gauged river sites on the San Juan River, Animas River and La Plata River recorded flows above bankfull in early June. No flood stages were reached. Monthly precipitation during the snow accumulation season changed drastically. Most of the precipitation fell in January with 230% of average realized. On the other hand, February and March saw a drop in precipitation to below average (75%) and much below average (40%), respectively. Precipitation increased in April (150%) and May (135%) returning to much above average. However, the water year precipitation for January through May remained much above average at 175%, 155%, 135%, 140% and 140% of average.
On February 1st , the average basinwide snowpack was at its highest (180%), dropping to 145% on March 1st . By April 1st, the snowpack dropped to 115% of average, increasing to 145% on May 1st , and , by June 1st , it was back down to 115%.
Most of the February and March forecasts overestimated the April-July observed flows. Early heavy snowpack and above average precipitation contributed to the higher forecasts. Warm temperatures in March increased streamflow early in the season, which was not reflected in the April-July forecasts. The April forecasts were dropped due to below average precipitation and warm temperatures, which melted the snowpack. The forecasts were increased again in May when precipitation and snowpack increased to much above average. The June forecasts remained nearly the same or decreased slightly at most forecast sites. Interestingly, the January most probable forecasts or 50% exceedance probability estimates were quite accurate considering the uncertainty involved in early forecasts. Most of the June forecasts underestimated the observed April-July runoff volumes , but remained within the reasonable maximum and minimum confidence boundaries.