Klamath River Basin hydrologic conditions prior to the September 2002 die-off of Salmon and Steelhead

Abstract More than 33,000 salmon and steelhead died in the lower Klamath River in late September 2002 on their way to spawning areas upstream. According to the California Department of Fish and Game, the cause of death was infection by protozoan and bacterial pathogens. Two factors that may have contributed to the disease incidence are low streamflow and high water temperature. September streamflows throughout the Klamath Basin were low, among the four lowest September flows recorded on the main stem since 1960. The low streamflows were caused by below-average snowpack and long-term drought, with resulting reduced ground-water discharge to streams. On the basis of historical climate data from the Klamath Basin and historical water tempera- ture data from an adjacent basin, September 2002 water temperatures were above the long-term average. Temperatures in the Klamath River above the fish die-off reach exceeded 65 degrees Fahren- heit for nearly all of September; multiple days of exposure by fish to temperatures at or above that level can greatly increase disease incidence. This report characterizes streamflow and water temperature conditions during the period leading up to the die-off and compares them to historical conditions in the Klamath River. This report is not an exploration of the causative mechanism of the die-off; rather, it is intended to provide detailed documentation of these conditions to be used by those examining the cause(s) of the die-off and to provide information that can contribute to decisions about future water management in the Klamath Basin.