Streamflow Forecast

The April 1 spring-summer streamflow forecast for the Southwest shows a 50 percent chance that all basins in the Colorado River, Rio Grande, and Arkansas watersheds will be below average (Figure 12). In many basins, forecasts call for flows to be less than 50 percent of average, including the Upper Colorado River and Rio Grande.

In Arizona, there is a 50 percent chance that the Salt, Verde, and Gila rivers will have streamflows equal to or less than 28, 41, and 7 percent of the February–May average, respectively. These values reflect the persistence of dry conditions since January 1, which were influenced by a La Niña event that contributed to snow and rain accumulations of less than 50 percent of average in many areas.

Winter precipitation in New Mexico was slightly more frequent than in Arizona and delivered more rain and snow, but most streamflow forecasts still project below-average flows. There is a 50 percent chance that the March–July flow in the Rio Grande will be less than or equal to 44 percent of average.

All snow-monitoring stations in the Upper Colorado River Basin are reporting below-average snowpacks, with most measuring less than 50 percent of the historical average. Many stations have been persistently below average this winter. As a result, spring inflow to Lake Powell is forecast to be only about 3.5 million acre-feet, or 44 percent of the 1971–2000 April–July average. This is a substantial decrease from forecasts issued on March 1, which called for inflow to be about 5.3 MAF. Combined water storage in Lakes Mead and Powell will almost assuredly decrease this spring. However, 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.