Gemming in beds of rivers is less common and require the assistance of experienced dredgers. An obstruction is put up across a stream to increase the flow of water at a selected spot over its bed. Long handled showels up to 40 feet or more in length are employed by six to eight men to drag the river bed at a point upstream of the dam till the illam layer is exposed after some weeks work. The overburden is carried away by the agitated water and the coarse sand and gravel (illam) is raked up and allowed to collect in a low ridge. This partly sorted illam is then removed from the water in baskets to the bank of the stream and the usual process of gem washing completed.