Two indices based on the equilibrium water temperature (Teq) are proposed to characterize the spatialdistribution of water temperatures in a river basin. One index is the deviation (Tw?Teq) between the annualaverages of the observed temperature (Tw) and Teq, and the other is the gradient (dTw/dTeq) obtained fromthe regression line with their monthly averages. When these indices were applied to three rivers in theChugoku region of Japan, the gradient showed a clear increasing trend from the headwaters to the rivermouth (or confluences of the main river in the case of tributaries) with little influence of river structures.In addition, the gradient could detect effects such as spring water and water released from dams, whichhave low seasonal variations in temperature. The deviation showed a decreasing trend along to the rivermouth, but in some cases, it increased when Teq changed more than Tw. |