## 2 Materials and Methods  
### Study Area  
Our study was situated within the Grand River Watershed (GRW), which is located in Southwestern Ontario, Canada (Figure 1a). The watershed covers an area of approximately 7000 kmcontaining the major urban centers of Kitchener, Waterloo, Guelph, Cambridge, and Brantford. Settlement in the GRW occurred in the early 1800s along with the onset of deforestation and agricultural practices. Agricultural practices intensified from the 1960s onward with the introduction of synthetic fertilizers and pesticides. Contemporary land use patterns in the GRW continue to be dominated by agriculture, covering approximately 80% of the landscape, with fragmented, natural vegetation covering about 19% of the total area [30]. Major agriculture types in the watershed include a combination of cash crops (_i.e._, soybeans and corn) and livestock operations (_i.e._, dairy cattle, beef, hog, and poultry) [31]. Climate in the GRW is temperate, with the central area (43.45\({}^{\circ}\) N, 80.38\({}^{\circ}\) W) having an average annual temperature, precipitation, and relative humidity of 7.0 \({}^{\circ}\)C, 916.5 mm, and 87.8%, respectively [32]. Figure 1: Map showing the location of the Grand River Watershed within the Great Lakes region of North America (**a**); and locations of the 13 headwater study catchments (grey) within the Nith and Conestoga subwatersheds of the Grand River Watershed (**b**). Map showing examples of the distributions of analyzed BMPs (Yellow: manure storage structure, Purple: livestock access restriction fence, and Red: erosion control structure) within study catchments (**c**). Orange surface areas represent agricultural land use. Site Selection and BMP Description  
For our study, 13 sampling sites were established on small agricultural streams within the headwaters of the Nith and Conestoga subwatersheds (Figure 1b,c). Both subwatersheds are characterized by an agricultural landscape (~83% agriculture), till soils (~76% of the soils), and a flat to rolling topography [33]. Study sites were selected to incorporate regional variation in BMP employment and comparable physiography and land cover among the catchments. Selected catchments were limited to those with areas between 5 and 12 km\({}^{2}\), till based soils (>65%), low natural (~25%) and urban (<5%) land covers, high agricultural (>85%) land use, locations accessible for sampling, and those with streams unimpeded by dams or other structures. The streams selected for sampling have not been subject to routine chemical or ecological monitoring in the past and thus historical or pre-BMP data was not available. This study focused on four structural BMPs: (1) manure storage systems (MS); (2) livestock access restrictions (LAR); (3) erosion control structures (EC); and, (4) riparian buffer areas (RBA) (Table 1). Structural BMPs have the advantage of being able to be strategically located on the landscape in hydrologically connected areas to maximize mitigation benefits. Furthermore, these BMPs are readily identifiable on the landscape using orthoimages. BMPs in the study area were predominantly implemented through collaboration between landowners and the Rural Water Quality Program administered by the Grand River Conservation Authority (GRCA). BMPs in this database were located in the study catchments using a Geographic Information System (GIS). Additional BMPs implemented by individual farmers were identified using high-resolution (0.5 m) orthoimages [34]. Sampling sites encompassed the full range of BMP abundance and location of the four different BMPs present in the region. A set of BMP metrics were calculated for each of the 13 selected catchments based on the type and location of BMPs present in the catchment (Table 2). Abundance descriptors included # of BMPs per farm, % RBA, length of LAR, % river with LAR, and % river protected. Location was characterized by hydrologic distance to stream, hydrologic distance to sampling point, flow accumulation, and % flow accumulation.