If the forest on the hill is cleared, than the soil will lose its natural cover and protection from erosion. The soil will become much more exposed to the erosion, and its top layers will be moved away relatively easy and quickly. Those layers will move downwards, and practically will end up into the river that passes beside the hill. With a significant increase of sediments getting into the river, the river will start to deposit much more material. Also, the sediments will make a problem when there's heavy rainfall because the river will be able to get out of its river bad more easily and cause a flood. The water of the river will also became murkier, muddier, which will make it dark yellowish/light brownish in appearance when there will be rainfall.