Sediment in Irrigation and Drainage Waters and Sediment Inputs and Outputs for Two Large Tracts in Southern Idaho

Sediment inputs from Snake River irrigation water and sediment losses back to the river were measured for two large irrigated tracts in southern Idaho. There was a net sediment accumulation of 0.69 metric tons/ha onto the 65,000 ha Northside tract but a net sediment loss of 0.46 metric tons/ha from the 82,000 Twin Falls tract. Differences in sediment losses from the two tracts result from the difference in sedimentation in the drain-ways of the two projects. Sediment deposited in drains on the Northside tract amounted to 4.5 metric tons/ha compared to 0.95 metric tons/ha for the Twin Falls tract. Drains on the Northside tract were constructed to grade whereas most drains on the Twin Falls tract are natural channels with steeper gradients. The net amounts of sediment eroded from farms within each tract were 4.0 metric tons/ha for the Northside tract and 1.42 metric tons/ha for the Twin Falls tract. This erosion loss from farms could be reduced within each tract by more careful use of water and construction of on-farm sediment retention ponds. This would also reduce the amount of sediment returned to the river and lower costs of mechanically removing sediment from drains and canals. Construction of sediment retention ponds along main drains and reducing the amount of surface runoff returning to the river would also reduce the amount of sediment returning