Waves and seiches in Flathead Lake, Montana: measurements of a quiet lake by differential travel-time current measurements

Measurements of velocity and pressure at the south end of Flathead Lake, Montana, reveal lake level fluctuations due to wind set up and seiches. Short period wind wave events, typically of single day duration, were recorded several times during the study period in the fall of 2004. The wind waves are fetch limited, in most cases with periods below 3 second but with significant wave heights that can exceed 50 cm during episodic storm events. Pronounced diurnal and longer period changes due to river influx and controlled drawdown through a hydroelectric dam across the exit river are clearly evident in the record. Broad scale upward fluctuations in lake level due to lake level regulation and seiches have exposed fragile portions of the shore to wind waves that have caused extensive lake wide erosion. The background motion of the lake is typically less than 2 cm/s, the waves are often relatively small with short periods, and the water is clear, making this a difficult environment for current and wave measurement. MAVS (modular acoustic velocity sensor) acoustic differential travel-time measurements detected and recorded these small signals cleanly and well above the instrument noise level.