Spatial patterns and temporal trends in water quality in Idaho’s lower Boise River and its tributaries, 1994–2023

The lower Boise River in southwestern Idaho is a vital cultural, economic, and ecological resource, but some of its beneficial uses are impaired by excess algae, sediment, nutrients, and bacteria. In response, a variety of water quality improvement projects and regulations have been implemented in recent decades. A recent study showed that, from water years 2003 to 2021, concentrations of total phosphorus at the mouth of the Boise River near Parma decreased by 60 percent, indicating the success of the improvement projects and progress towards reaching the regulatory target. However, although the progress of the system as a whole towards reaching the total phosphorus target is well documented, quantifying contributions and trends in total phosphorus loading from individual tributaries and upstream mainstem locations is needed. Similarly, tributary contributions and trends of other constituents of concern are unknown or have not been evaluated in a decade. To evaluate the success of past water-quality improvement efforts and help prioritize future investments, this study characterized spatial and seasonal patterns in concentrations and loads of suspended sediment, total phosphorus, orthophosphate, total nitrogen, nitrate plus nitrite, Escherichia coli bacteria, and periphyton chlorophyll a (chl-a) at three mainstem and seven tributary sites of the lower Boise River during water years 2019–2023. For sites and constituents with adequate data, temporal trends over a 15-year and 30-year period were also evaluated.

Recent (in other words, water years 2019–2023) concentrations and loads of suspended sediment and total phosphorus were typically greatest during the irrigation season and often exceeded total maximum daily load targets. However, concentrations and loads of suspended sediment, total phosphorus, and orthophosphate have decreased over the past 15 to 30 years at many sites. In contrast, concentrations of total nitrogen and nitrate plus nitrite showed increasing trends at the Boise River near Parma, likely reflecting the effects of regional changes in agricultural practices in recent decades. Periphyton chl-a concentrations during October–November were highly likely increasing over the 30-year period at Boise River at Eckert, but were uncertain at Middleton and Parma. At Boise River at Middleton, periphyton chl-a concentrations exceeded the total maximum daily load target of 150 milligrams per square meter in 75 percent of samples during water years 2019–2023. Results therefore demonstrate that past watershed improvement efforts have been largely effective at reducing concentrations and loads of suspended sediment, total phosphorus, and orthophosphate in the lower Boise River watershed, but different strategies may be needed to reduce concentrations and loads of total nitrogen, nitrate plus nitrite, Escherichia coli, and periphyton chl-a.