Streamflow Forecast

The March 1 spring-summer streamflow forecast for the Southwest shows a 50 percent chance that all but one basin in the upper and lower Colorado River and Arkansas watersheds will be below average (Figure 12). Only a few basins in the Rio Grande are expected to have above- or near-average flows.

In Arizona, the likelihood that the Salt, Verde, and Gila rivers will have streamflows of 37, 31, and 27 percent of the February–May average, respectively, is 50 percent. Although widespread and copious rain and snow fell in many basins in mid-March, this boost likely will not substantially increase spring streamflows. The La Niña helped deliver a dry spell that extended from late December to mid-March. This protracted period has all but ensured below-average streamflows for most of the Southwest, unless powerful and frequent late winter storms soak the region.

Winter precipitation in New Mexico has been more frequent and has delivered more rain and snow than in Arizona, but most streamflow forecasts also project below-average flows. There is a 50 percent chance that the March–July flow in the Rio Grande will be between 70 and 89 percent of average. On the other hand, near-average flows are expected in the Canadian River in northeast New Mexico. 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 67 percent of the 1971–2000 average for April–July, or about 5.3 million acre-feet. This is a slight increase from forecasts issued on February 1 but is about 2.5 million acre-feet below average. The forecast also indicates only about a 10 percent chance that Lake Mead inflow will be above average. Last winter’s exceptionally high streamflows, which delivered about 7 million acre-feet more than average to Lakes Mead and Powell, will help buffer below-average flows in the Colorado River this year.