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.