Idaho Falls Subbasin

Three stream segments in the Idaho Falls subbasin are listed on the §303(d) list. The hydrology of the Idaho Falls subbasin is dominated by the Snake River and its associated diversion structures for irrigation of farmland on the Snake River plain.

Flow in the South Fork Snake River is controlled upstream of the subbasin by Palisades Reservoir. Numerous irrigation diversions also influence flow on the South Fork Snake River. A small section of the South Fork Snake River at the eastern-most border of the subbasin is §303(d) listed for flow alteration. However, flow alteration is not considered a “pollutant” under the Clean Water Act, and TMDLs are not required for pollution that isn’t caused by a “pollutant.” However, it is recommended that this stream reach remain on the §303(d) list for flow alteration.

South Fork Willow Creek has been §303(d) listed for sediment; however, this stream no longer exists as a natural watercourse. Since the construction of Ririe Dam in the 1970s, flow in the Willow Creek/Sand Creek complex has been controlled for irrigation. Willow Creek, including both the North and South Forks, has been converted to canal conveyance structures with straightened channels and rip-rap style bank reinforcement. No water flows in these channels during the nonirrigation season. Therefore, it is recommended that South Fork Willow Creek be removed from the §303(d) list.

Birch Creek was added to the 1998 §303(d) list with unknown pollutants. A subsequent investigation revealed that the primary water quality problem is likely sediment from bank erosion. Birch Creek is in a predominantly dryland agricultural region and is constrained between a road and agricultural fields. No data were available for Birch Creek; therefore, a TMDL for sediment was constructed by using the adjacent Antelope Creek TMDL as a proxy. Because of similar geology, soils, and land use, loading analyses from Antelope Creek will suffice until such time that erosion surveys can be completed for Birch Creek.