Irrigation return flow sourcing of sediment and flow augmentation in receiving streams: A case study

ABSTRACT: The total maximum daily load (TMDL) approach to defining and quantifying point and nonpoint sources of impairment to surface water systems provides an analytical approach to assigning acceptable levels of allowable impairment from various sources. However, in streams affected by irrigation return flows, identifying actual origins and causes of nonpoint flow and sediment production and quantifying those contributions for use in TMDL planning is a challenging task. It has been hypothesized that Muddy Creek, a tributary of the Sun River-Missouri River watershed in west central Montana, is significantly impacted by irrigation return flows. A two-year case study of Muddy Creek gathered and analyzed stream flow and sediment data in its tributaries and main channel. Flow and sediment were partitioned among possible sources, revealing that 49 to 67 percent of flow delivered to the Sun River by Muddy Creek during the irrigation season originated within irrigation district boundaries, while 58 to 72 percent of sediment entering the Sun River via Muddy Creek during the irrigation season originated downstream of the district boundaries within Muddy Creek9s 20 km (12.5 mi) lower reach. It was concluded that this added sediment load is a direct result of flow augmentation from return flows and operational spills within the irrigation district. This protocol resulted in acquisition of reliable, objective data and could be successfully applied to other watersheds. The data also demonstrate potential effects of irrigation return flow on flow volume and sediment load of receiving streams.