Effects of a temperature control device on nutrients, POM and plankton in the tailwaters below Shasta Lake, California

A temperature control device (TCD) was installed by the U.S. Bureau of Recl amation on Shasta Dam, California, in March 1997 for controlling downstream river temperatures. Temperature modification was required to aid recovery of the endangered winter run chinook salmon ( Oncorhynchus tshawytscha) in the Sacramento River, and to minimize loss of generating capacity as a resu lt of releasing deeper, colder water through low level outlet works to meet downstream temperature criteria. This study began two years prior to opera tion of the TCD, to compare pre- and post-operational changes on downstream tailwaters, including nutrients, particulate organic matter (POM) and plan kton. During epilimnetic withdrawals from January to mid-June, and mid-leve l withdrawals through August, operation of the TCD was associated with decr eases in dissolved nitrate-nitrate concentrations, localized increases in s mall particulate organic matter (SPOM) at Shasta tailwaters, increases of b acillariophyta (< 25 mum size fraction), and increases in copepod biomass. These changes can potentially influence the food base of the river and ther efore fish production in the Upper Sacramento River, including the chinook salmon.