Streamflow Forecast

The spring–summer streamflow forecast for the Southwest, issued on February 1, shows a 50 percent chance that all basins in the upper and lower Colorado River watershed and Arkansas basin will be below average (Figure 12). Only a few basins in the Rio Grande are expected to have above-average flows.

In Arizona, the likelihood that the Salt, Verde, and Gila rivers will have streamflows of 37, 40, and 49 percent of the February–May average, respectively, is 50 percent.

Although widespread and copious rain and snow soaked many mountain regions during November and most of December, dry and warm conditions have since largely prevailed. These conditions are expected to continue, mostly part because the La Niña event is expected to persist into spring. The La Niña also is influencing forecasts in New Mexico. There is a 50 percent chance that the March–July flow in the Rio Grande, measured at Otowi Bridge, and the Gila River, measured near Virden, will be 79 and 52 percent of average, respectively. On the other hand, near-average flows are expected in the Pecos River and above-average flows are forecast for the Mimbres River, largely because early winter snows dumped precipitation in these regions. Streamflow forecasts are issued every month for New Mexico and every two weeks for Arizona. The forecasts become progressively more accurate as the winter progresses.

In the Upper Colorado River Basin, spring inflow to Lake Powell is forecast to be about 64 percent of the 1971–2000 average for April–July, or about 3.2 million acre-feet. The forecast also indicates only 10 percent chance that Lake Powell inflow will be 108 percent of average. Last winter’s exceptionally high streamflows, which increased combined storage in Lakes Mead and Powell by about 7 million acre-feet between April and July—about 2 million acre-feet more than average—will buffer below-average flows in the Colorado River this year.