The effectiveness of small-scale headwater storage dams and reservoirs on stream water quality and quantity in the Canadian Prairies

Abstract

In response to flooding and soil erosion impacting the South Tobacco Creek watershed in southcentral Manitoba, local landowners constructed a network of small dams and reservoirs in the headwaters. Between 1999 and 2007, two of the small dams/reservoirs (Steppler multipurpose dam and Madill dry dam) were intensively monitored for their effectiveness in reducing peak flows and downstream sediment and nutrient loading during spring snowmelt (typically mid-March to mid-April) and summer rainfall (typically May to November) periods. These small-scale headwater storage dams were effective in reducing peak flows from agricultural land. The two dams/reservoirs monitored also reduced annual concentrations of sediment and total nitrogen (TN) to downstream receiving waters. However, annual concentrations of total phosphorus (TP) were only significantly reduced at the Madill dry dam, and the average concentrations of nitrogen (N) and phosphorus (P) within outflow water samples still exceeded guidelines for freshwater in the Canadian Prairies. Both dams/reservoirs significantly reduced annual loads of sediment, TN, and TP (Steppler dam, average of 77%, 15%, and 12%, respectively; Madill dam, average of 66%, 20%, and 9%, respectively). This corresponded to an average annual retention of 25 Mg y⁻¹ (28 tn yr⁻¹) of sediment, 166 kg N y⁻¹ (366 lb N yr⁻¹) and 17 kg P y⁻¹ (37 lb P yr⁻¹) by the Steppler dam, while 6 Mg y⁻¹ (7 tn yr⁻¹) of sediment, 181 kg N y⁻¹ (399 lb N yr⁻¹) and 10 kg P y⁻¹ (22 lb P yr⁻¹) were retained by the Madill dam. Both reservoirs reduced annual loads of dissolved N and P to downstream water bodies (Steppler, average of 14% and 10%, respectively; Madill, average of 23% and 15%, respectively), and were generally effective in removing dissolved N and P during both snowmelt and rainfall-generated runoff. The percent retention of dissolved nutrients was consistently higher during the summer than the spring. While the reservoirs removed particulates during snowmelt-generated runoff, they were often sources of suspended nutrients during rainfall-generated events. However, since dissolved nutrients were the dominant form of both N and P (>70% for both snowmelt and rainfall events), the two dams/reservoirs successfully reduced overall nutrient loads to downstream water bodies, annually and seasonally. In combination with improving flood and erosion control for the region, small headwater storage dams and reservoirs deserve consideration when developing watershed nutrient management plans, especially for undulating and hummocky regions on the Great Plains.