The Main Fork is an open river. There are no dams to slow or control the flow, and there are no diversions for agriculture. The Main Fork delivers 6 million acre-feet (MAF) in a year. The flow varies from month to month as shown on the graph. The flow in the winter months is similar to the average flow of 6 MAF/yr.

The flow increases during the Spring, and the April flow reaches 12 MAF/year. After the Spring runoff, the flow declines to low values for the summer months. The flow returns to the average value in November. The flow from the Main Fork combines with the North Fork flow before the river enters American Lake in the northeast part of the region.