Small Dams and Reservoirs: Effect On Stream Water Quality and Quantity in the Canadian Prairies.

For years, flooding and soil erosion would regularly impact the agricultural land surrounding the South Tobacco Creek watershed in south-central Manitoba.  In response, local landowners constructed a network of small dams and reservoirs in the headwaters of this watershed for erosion and flood control.  Between 1999 and 2007, two of the small reservoirs (Steppler and Madill) were intensively monitored for their effectiveness in reducing flood peaks and downstream sediment and nutrient loading during spring snowmelt and summer rainfall periods. 

The two reservoirs successfully attenuated peak flow as runoff waters were routed through the reservoirs, but little of the overall runoff volume was retained for a significant length of time.  Both reservoirs significantly reduced annual loads of sediment, nitrogen (N) and phosphorus (P) – Steppler, average of 77, 15 and 12 % yr^-1, respectively; Madill, average of 66, 20 and 9 % yr^-1, respectively.  They were also effective in reducing annual loads of dissolved N and P to downstream water bodies (Steppler, average of 14 and 10 % yr^-1, respectively; Madill, average of 23 and 15 % yr^-1, respectively), during both snowmelt and rainfall-generated runoff periods.  While the reservoirs removed particulates during snowmelt-generated runoff, they were often sources of suspended nutrients during rainfall-generated events.  We suspect that this was a combination of relatively few particulates in the inflow water and algal growth during the summer.  However, since dissolved nutrients were the dominant form of both N and P (> 70 % in both snowmelt and rainfall-induced runoff events), the two reservoirs were successful in reducing overall nutrient loads to downstream water bodies, annually and seasonally.  In combination with improving flood and erosion control for the region, the concept of small dams/reservoirs has sufficient value that deserves consideration when developing watershed management plans, especially for agricultural escarpment regions, on the Great Plains.