Dynamic response of stream temperatures to boundary and inflow perturbation due to reservoir operations

Dams and reservoir operations modify natural stream behaviour and affect the downstream characteristics such as mean temperatures and diurnal temperature amplitudes. Managing phase effects due to reservoir operation and the associated amplification of daily maximum temperatures in the downstream reaches remains a challenge. An analytical approach derived from a one-dimensional heat advection and dispersion equation with surface heating in the form of equilibrium temperature was developed to examine the potential for restoration of natural stream temperatures. The analytical model was validated with observed temperature data collected in the Clackamas River, Oregon, and was used to highlight key downstream temperature behaviour characteristics. Mean stream temperatures below the dam are relatively stable and upon deviating from natural stream mean temperatures, return asymptotically to their natural state. In contrast, the amplitudes of daily temperature variation are highly sensitive to the phase differences induced by the dam and could nearly double in natural amplitude within the first 24 h. The analysis showed that restoring average stream temperatures to natural levels through structural and operational modifications at the dam may not be sufficient as phase-induced temperatures maximums would continue to persist. Copyright (C) 2008 John Wiley & Sons, Ltd.