Rockland basin comprises about 320 sq mi of the Snake River drainage in southeastern Idaho. Mountain ranges bordering the basin are composed predominantly of limestone and are complexly faulted. Major aquifers include Holocene alluvium, Quaternary-Tertiary volcanic rocks, and Tertiary sedimentary rocks. Groundwater occurs under water table conditions except where it is locally confined. Groundwater discharges to springs in the Deep Creek Mountains and maintains perennial streamflow. Near the mouth of Rock Creek, groundwater movement is northward toward the Snake River. Underflow is estimated to be 51,000 acre-ft/yr.
Total water yield available to Rockland basin is estimated to be 5.0 in. (85,000 acre-ft) of the estimated 17.3 in. of annual precipitation. Evapotranspiration ranges from 9.9 to 17 in./yr, depending, in part, on altitude of the land surface.
An estimated 12,000 acre-ft of surface water and 3,500 acre-ft of groundwater are used annually for irrigation. Less than 100 acre-ft of water is used for public supply, domestic, and stock supplies. East Fork Rock Creek supplies the most surface water for irrigation of agricultural lands.
At the present (1980) state of groundwater development in Rockland basin, streams and aquifers are hydraulically connected. Pumping of groundwater in increased quantities from wells near streams will affect groundwater movement and may diminish streamflow. There are no long-term regional water table declines at present. Continued water level monitoring of selected wells may aid in documenting effects of future management practices on the groundwater system.
