Data in these tables for streams with major reservoirs have been adjusted by adding upstream reservoir change-in-storage volumes and major diversions to the observed monthly streamflow volumes to reflect a "natural flow" condition. Reservoir evaporation and most diversions are not accounted for in the NRCS database listed here. The U.S. Geological Survey (USGS), U.S. Bureau of Reclamation (USBR) and Idaho Department of Water Resources (IDWR) are a source of most of the data presented. Streams without reservoirs and stations upstream of the reservoirs show monthly data as received directly from USGS archived records.
Measuring the amount of water held in the mountain snowpack in a given basin provides a better understanding of water supply and, in some cases, a better understanding of potential runoff volumes during the spring and summer months. Indexing the snow water values allows for easy comparison from year to year, and for observing trends over time. Daily measurement data are recorded at SNOTEL sites in or near the watershed divide for each basin. Monthly indexes are calculated by summing first of the month values of snow water content for each of the long-term sites in the basin.