The Elements of Runoff - Mountain snow water equivalent for Upper Missouri River Basin Runoff - Mountain snow begins to accumulate in October, peaks in April and snowmelt is usually complete by mid-July. The June 1 runoff forecast is 26.8 million acre feet. Warm temperatures led to a quick melt of the mountain snowpack. The rapid melting combined with precipitation in the western half of the Basin resulted in above-average inflow into the reservoir system.