Introduction
As of 2018, over 55% of the global population lives in cities, with nearly 25% living in a city over 1 million people. By 2030, the United Nations predicts that over 60% of the global population will be urban and this number will likely continue to rise for the foreseeable future (United Nations, 2018). Consequently, to understand ecological systems in our anthropogenic world, it is critical to consider the influence of densely populated areas on these systems. Humans fundamentally change natural systems in a variety of ways including physical alterations, changes in energy flow, and disruption of biogeochemical cycles. Due to the overwhelming size of modern cities and the increasing demand for resources, hydrologic systems are severely impacted (Millennium Ecosystem Assessment, 2005).
Within a waterway, the flow regime is a measure of the pattern and variation in streamflow over time and includes the magnitude, frequency, duration, timing, and rate of change of hydrologic conditions in a river. In a natural system, the flow regime is controlled by a range of parameters including climate, geomorphology, soils, biota, watershed size, and stream pattern (Poff et al., 1997). In the urban environment, physical modifications including dam construction and impervious surface can fundamentally alter the natural flow regime (Baker, Richards, Loftus, & Kramer, 2004).
Dams serve a variety of purposes, but two of the most common are flood control and water storage (Graf, 2001). By regulating discharge from the reservoir, managers can reduce the magnitude of high flow events and minimize the risk of downstream flooding. Often, water is removed directly from the reservoir for use, thus reducing flow in the downstream river. Low flow from dam restriction can partially explain some of the water degradation in urban areas; with reduced flow, pollutant concentrations may be elevated due to a lack of dilution (Mosley, 2015; Olatunde et al., 2015; Rolls, Leigh, & Sheldon, 2012).
Perhaps the most significant aspect of the urban landscape that affects waterways is increased impervious land cover. Urban catchments are dominated by large, impervious surface areas that reduce infiltration and funnel water into engineered systems designed to convey water quickly and efficiently (Booth & Jackson, 1997). These engineered drainage areas tend to result in flashy flows, with flowrates dramatically affected by even small precipitation events. In addition, rapid sheet flow leads to the accumulation of nutrients, heavy metals, sediment, and other pollutants and transports them to the river (Paul & Meyer, 2001; Rolls et al., 2012; Walsh et al., 2005).
In many urban areas, altered hydrology contributes significantly to the overall water quality in urban rivers. Elevated concentrations of nitrogen, especially ammonia (NH3), nitrate (NO3-), and nitrite (NO2-) (Groffman, Law, Belt, Band, & Fisher, 2004; Larned, Snelder, Unwin, & McBride, 2016; Schoonover, Lockaby, & Pan, 2005); and phosphorus, including total phosphorus (TP) and orthophosphate (PO43-) (Carpenter et al., 1998; Nagy, Lockaby, Kalin, & Anderson, 2012; Zhang, Shao, Liu, Xu, & Fan, 2015) have been observed in many urban areas. In addition to surface runoff, wastewater treatment plant effluent, raw sewage, fertilizer, and fossil fuel combustion (atmospheric deposition) are common sources of nitrogen and phosphorus (Bernhardt, Band, Walsh, & Berke, 2008; Brown et al., 2009; Gregory & Calhoun, 2007; Son, Goodwin, & Carlson, 2015).
Biochemical oxygen demand (BOD) is often used to assess the amount of organic pollution in a body of water. In urban areas, elevated BOD can be attributed to a variety of causes including discharge from wastewater treatment facilities, effluent from pulp and paper mills and food processing plants, dead plants and animals, and raw sewage (Magdaleno et al., 2001; Olatunde et al., 2015; Rice & Bridgewater, 2012). When BOD is high, dissolved oxygen (DO) is often low, although DO can also be affected by temperature, the presence of ammonia, and the degree of mixing due to water movement (Rice & Bridgewater, 2012). Low DO concentrations are common in urban waterways and can significantly affect the health of aquatic biota (Glinska-Lewczuk et al., 2016; Olatunde et al., 2015; Ouyang, Zhu, & Kuang, 2006).
Urban areas in general tend to have higher air temperatures due to the urban heat island affect. This phenomenon, in addition to stormwater runoff, effluent from electricity generation, and effluent from industrial processes can contribute to elevated temperatures in urban streams (Abdi & Endreny, 2019; Herb, Janke, Mohseni, & Stefan, 2008; Somers et al., 2013). Water temperature is one of the most important factors influencing the aquatic biotic community. Restoring native biota to an urban river is often challenging due to the thermal characteristics of the system (Cockerill & Anderson, 2014; Wang, Lyons, & Kanehl, 2003).
Water in urban systems often departs from neutral and, in some cases, it can be strongly acidic or basic (Pasquini, Formica, & Sacchi, 2012; Peters, 2009; Szita et al., 2019). Industrial pollutants, atmospheric pollution (deposition), and underlying geology (carbonate minerals) can affect pH in the urban environment (Malmqvist & Rundle, 2002; Peters, 2009). Acidification of urban rivers is directly detrimental to biota and is also correlated with increased concentrations of heavy metals (Das, Nordin, & Mazumder, 2009).
While it seems clear that urban hydrology tends to affect water quality in a negative way, it is less clear how these effects are impacted over space and time. In this study, we measured a range of water quality parameters in the South Platte River, which flows approximately 60 km though the Denver metropolitan area, Colorado, USA. We had two goals: 1) To measure water quality over several years to determine whether seasonal hydrologic patterns influence water quality and 2) To evaluate how the urban drainage system and the amount of impervious surface area affected water quality in this system.