The physical limnology of Winam Gulf and Rusinga Channel of Lake Victoria during April-May and August of 2005

Winam Gulf (Kenya) is a large (surface area ~ 1400 km2) and shallow (<20 m) bay of northeastern Lake Victoria with only one connection to the open lake through Rusinga Channel. To understand the exchange dynamics between Winam Gulf and the offshore waters of Lake Victoria and the hydrodynamics of the region, field studies were carried out from Apr. 22-May 4 and Aug. 5-16 of 2005. A meteorological station (shortwave, total radiation, air temperature, relative humidity, wind speed and direction), thermistor chain (0.75m vertical resolution) and ADCP (40 cm vertical resolution) were deployed in Rusinga Channel in a depth of 20m. Similarly, at an offshore station in northeastern Lake Victoria another thermistor chain was deployed in a water depth of 40 m along with wind speed and direction sensors. Over both field campaigns the exchange dynamics through Rusinga Channel behaved similar to a tidally driven system with surface level fluctuations of between 5-15cm at the ADCP location, and much larger excursions at the eastern end of Winam Gulf. In general, these surface level movements led to barotropically driven flows into the Gulf during rising surface levels and currents towards the open lake during falling lake level. The frequency of these currents was found to vary between 6 and 12 hours and current speeds ranged from 10-50 cm s-1. Field data and ELCOM simulations indicate that despite the high current velocities in the channel the net exchange is low due to the oscillatory nature of the forcing. This implies that the Gulf is relatively decoupled from the main lake.

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