Stream temperature variation in regulated rivers: Evidence for a spatial pattern in daily minimum and maximum magnitudes

Elevated water temperatures have become an important concern in watersheds where aquatic species such as salmonids are present. To maintain cold water downstream of large reservoirs, releases sometimes are taken from beneath the thermocline where water temperature exhibits little diurnal variation. This unusual upstream boundary condition, that is, steady flow and temperature, causes a characteristic pattern of nodes of minimum diurnal variation and antinodes of maximum diurnal variation to form at regular intervals downstream of the regulating reservoir. Within this paper, an analytical model is derived in order to illustrate the phenomenon of nodes and antinodes. Field data as well as analytical and numerical model simulations illustrate the formation of nodes and antinodes over a 250 km reach of the Sacramento River, located in the Central Valley of California.