Download An Assessment of Stream Discharges and Sediment Load and their

LARS 2007
Catchment and Lake Research
An Assessment of Stream Discharges and Sediment Load and their
Implications to the Environmental Health on Mt. Elgon, Eastern Uganda
Bamutaze, Yazidhi
Makerere University, Department of Geography
Abstract
The Mt. Elgon watershed in Eastern Uganda is undergoing environmental degradation, the most
serious being soil erosion. This degradative process results in colossal economic losses through
the various on-site damages especially reduction in land productivity. Unfortunately, the
problems and damages transcend the onsite effects and are exported to the low land areas of Mt.
Elgon and in the streams. For example it is a common feature to observe brown to reddish brown
colored waters of rivers and streams Mt. Elgon, a visible manifestation of high concentration of
sediment believed to be eroded from the farm lands. The gist of this study was to assess stream
discharge and sediment load and relate to soil loss levels in the mountain catchment. Two
Parshall flumes fabricated locally were calibrated and installed at the upper and lower reaches of
the major stream draining Wanale micro-catchment where detailed erosion and land use studies
were undertaken. Stream discharge measurements were taken and recorded before and after each
study rainstorm using a graduated ruler that was dipped in water at a specific point in the flume.
At the onset of the rainstorm, measurements were taken at intervals of 15 minutes for 1 hour as
the stream water volume increased and color changed to brown then after very 30 minutes until
when the stream volume declined. In addition, samples for sediment load and water quality
determination were taken. Suspended and bed load sediments samples were taken using standard
methods from the streams at the flume sites. The results showed that discharge and sediment
load of streams increases downstream as well as with time to a peak following the on-set of a
rainstorm. The base flow (before rainstorms) at the lower and upper stream reaches were 40 and
30cm, respectively. Peak discharge at the lower and upper gauging stations was 90 and 75 cm,
respectively, and was attained between 30 and 45 minutes from the onset of the rainstorms. The
total suspended sediment (TSS) for data taken on 12/11/2002 indicate increases with discharge
(rising limb of hydrograph) to peaks of 80 and 61 mgTSS/L of water at 105 and 500o C,
respectively. Results of TSS taken on 14/01/2003 and 16/01/2003 even reveals higher
magnitudes for the lower stream gauging station. These observations dovetail the runoff and soil
los measurements at plots requiring concerted conservation efforts for the sustainability of the
Mt. Elgon watershed.
Key words: Erosion, sedimentation, Mt. Elgon
30
Catchment and Lake Research
LARS 2007
Notes:
31