Investigating impacts of small dams and dam removal on dissolved oxygen in streams

Abstract

Small surface-release dams are prevalent across North American watersheds and can alter stream flow, thermal regimes, nutrient dynamics, and sediment transport. These dams are often implicated as a cause of negative water quality impacts-including reduced dissolved oxygen (DO)-and dam removal is increasingly employed to restore natural stream processes and improve DO. Published impacts of small dams on DO vary widely across sites, and even less is known about the extent and timescale of DO recovery following removal. Therefore, we sought to quantify the effects of small dams and dam removal on DO and determine the dam, stream, and watershed characteristics driving inter-site variation in responses. We deployed continuous data loggers for 3 weeks during summer months in upstream (reference), impoundment, and downstream reaches at each of 15 dammed sites and collected equivalent data at 10 of those sites following dam removal. Prior to dam removal, most sites (60%) experienced a decrease in DO (an average of 1.15 mg/L lower) within the impoundment relative to upstream, but no consistent impacts on diel ranges or on downstream reaches. Before dam removal, 5 impacted stream reaches experienced minimum DO levels below acceptable water quality standards (<5 mg/L); after dam removal, 4 of 5 of these reaches met DO standards. Sites with wider impoundments relative to upstream widths and sites located in watersheds with more cultivated land experienced the greatest decreases in impoundment DO relative to upstream. Within one year following dam removal, impoundment DO recovered to upstream reference conditions at 80% of sites, with the magnitude of recovery strongly related to the magnitude of pre-removal impacts. These data suggest that broadly, small dams negatively affect stream DO, and the extent of effects are modulated by impoundment geometry and watershed characteristics. These results may help practitioners to prioritize restoration efforts at those sites where small dams are having outsized impacts, and therefore where the greatest water quality benefits may occur.

Fig 1. Study sites.

Map of the 15 dams in Massachusetts, USA, monitored within this study, of which 10 (circles) were removed over the course of this study and 5 (squares) remained standing. See Table 1 for site information corresponding to numbers. Map was prepared using ArcGIS 10.8 (Esri, Redlands, California, USA).

Fig 2. Dissolved oxygen (DO) effects before and after dam removal.

(A) Variation in impoundment effect (mean daily impoundment minus upstream DO) by site before and after dam removal. (B) Variation in downstream effect (mean daily downstream minus upstream DO). Error bars represent one standard deviation about the mean. The horizontal line at zero indicates no difference between impoundment or downstream DO and the upstream reference. Significant differences (α = 0.05) between DO effects before and after dam removal are annotated with an asterisk (*) and sites where the dam was not removed during this study are annotated with an X. Sites are ordered from lowest to highest pre-removal impoundment effect.

Fig 3. Magnitude and direction of Cliff’s delta effect size, which was used to assess the dam removal dissolved oxygen (DO) response in A) the impoundment, and B) downstream.

Positive values indicate an increase in mean impoundment or downstream DO relative to upstream after dam removal. Gray bars indicate 95% confidence interval. Cliff’s delta values can be interpreted as negligible (|<0.147|), small (|<0.33|), medium (|<0.474|) and large (|0.474–1.000|) effects. Sites are ordered from lowest (bottom) to highest (top) pre-removal impoundment effect.