Study Area
The South Platte River presents a unique opportunity to investigate the influence of urbanization on water quality. It emerges from the Rocky Mountains in the southwest corner of the Denver metropolitan area and travels more than 60 km northeast through the length of the city (Figure 1). There is little development upstream of the emergence point, although abandoned mines may affect water pH and contribute some heavy metals and sediment. As the river flows through the metro area, it moves through several different levels of urbanization. It begins by moving through suburban areas for approximately 30 kilometers, then goes directly through the highly urbanized core of the downtown area for approximately 10 km before moving again into suburban areas for approximately 10 km and finally through exurban, agricultural, and natural grassland areas for another 10 km.
The South Platte Watershed has an area of approximately 62,937 km2 and is located in three states: Colorado (79% of the watershed), Nebraska (15%), and Wyoming (6%) (Figure 2). The South Platte River originates in the Rocky Mountains in central Colorado, at an elevation over 4,300 m and travels approximately 725 km to its confluence with the North Platte River. The Platte River is a part of the Mississippi River watershed which ends in the Gulf of Mexico (Dennehy, Litke, McMahon, Heiny, & Tate, 1995). Once the South Platte River enters the Denver metro area, it is designated as a warm water stream, meaning warm-water aquatic species are most commonly found in the river (Colorado Departmet of Public Health and Environment, 2017).
The basin includes two physiographic provinces, the Front Range Section of the Southern Rocky Mountain Province and the Colorado Piedmont Section of the Great Plains Province. Much of the geology underlying the Denver region consists of unconsolidated surficial eolian and fluvial deposits and sedimentary rock formations (Trimble, Machette, Moore, & Murry, 2003). Denver’s climate is semi-arid, with an average annual precipitation of 36.3 cm and an average annual temperature of 10.3oC. Most of Denver’s precipitation arrives between May and August, in the form of rain. (National Weather Service, 2020). Headwater areas to the west of Denver average 75 cm or more of precipitation, with most of it coming as snow (Dennehy et al., 1995).
Although urban land makes up less than 10% of the area in the basin, cities along Colorado’s Front Range are growing rapidly, putting strain on the South Platte River. Over 70% of Colorado’s 5.7 million people live in the basin, including approximately 3.2 million people that live in the Denver metropolitan area alone (Colorado Departmet of Public Health & Environment, 2018; United States Census, 2020). The Colorado State Demography Office (2020) projects the Denver metro population will surpass 4 million people by 2050.
The South Platte River is a major source of water for the Front Range, providing more than 50% of the water used by Denver metropolitan municipalities (Denver Water, 2020). Within the Denver metro area, most of the river flow is diverted for domestic and industrial use and is returned to the river through wastewater effluent, which makes up the majority of the flow downstream of the city through much of the year (Metro Wastewater Reclamation District, 2020; Strange, Fausch, & Covich, 1999; Waskom, 2013). Before entering the Denver metropolitan area, the South Platte River travels through numerous dams, including the dam at Chatfield Reservoir on the southwestern edge of the city. Consequently, the magnitude of spring high flows have been somewhat minimized, although flow still peaks in late spring, diminishes through the summer and autumn and remains low through the winter. In addition, main channel flow is relatively consistent year-to-year making the impacts of urban runoff even more important (Figure 3) (Dennehy et al., 1998; United States Geological Survey, 2020; Waskom, 2013).
Elevated nutrient concentrations, pesticide residue, heavy metals contamination, elevated levels of E. coli , elevated temperature, low dissolved oxygen, and excessive sediment loading have been observed in the South Platte River. (Dennehy et al., 1998; Denver Environmental Health, 2016). In 2018, the South Platte River and all of its metro area tributaries appeared on the state of Colorado 303d list of impaired or threatened waters due to diminished water quality (Colorado Departmet of Public Health & Environment, 2018).