National Water-Quality Assessment Program Effects of Urban Development on Stream Ecosystems along the Front Range of the Rocky Mountains, Colorado and Wyoming By Lori A. Sprague, Robert E. Zuellig, and Jean A. Dupree The U.S. Geological Survey (USGS) conducted a study from 2002 through 2003 through its National Water-Quality Assessment (NAWQA) Program to determine the effects of urbanization on the physical, chemical, and biological characteristics of stream ecosystems along the Front Range of the Rocky Mountains. The objectives of the study were to (1) examine physical, chemical, and biological responses at sites ranging from mini- mally to highly developed; (2) determine the major physical, chemical, and landscape variables affecting aquatic communities at these sites; and (3) evaluate the relevance of the results to the management of water resources in the South Platte River Basin. How can urban development affect stream where refuge (seclusion and rest) areas such as boulders and woody debris are lacking (Winterbourn and Townsend, 1991). ecosystems? In addition, higher streamflows are associated with increased movement of sediment to streams, which can affect aquatic As land areas urbanize, stream ecosystems can be communities by decreasing light penetration and photosyn- substantially altered. Impervious surfaces—surfaces that are thesis and degrading stream-bottom habitat (Waters, 1995). impenetrable to water, such as parking lots, rooftops, and Higher contaminant concentrations and stream temperatures paved roads—can prevent rainfall from infiltrating into soil can adversely affect growth, reproduction, species competi- and ground water, leading to increased runoff to streams. tion, and disease progression within aquatic communities With rainfall moving to streams more quickly and in greater (Fitzgerald and others, 1999; LeBlanc and others, 1997). amounts, streamflow conditions can change more rapidly, the peak streamflows may be higher, and flooding may occur more frequently in urban areas (U.S. Environmental Protec- How did the U.S. Geological Survey study the tion Agency, 1997). Increased runoff to streams often leads to effects of urban development? changes in water quality as well. Runoff can transport con- taminants to streams from a variety of urban sources, including Most previous studies of stream ecosystems have focused automobiles (hydrocarbons and metals); rooftops (metals); on either pristine areas or highly developed areas; little is wood treated with preservatives (hydrocarbons); construc- known about how the gradual progression of urban develop- tion sites (sediment and any adsorbed contaminants); and golf ment between these two extremes affects stream ecosystems. courses, parks, and residential areas (pesticides, nutrients, To address this, the USGS conducted a study from 2002 bacteria) (Pitt and others, 1995). In addition, stream channels through 2003 through its NAWQA Program to determine the in urban areas can be straightened, deepened, and widened effects of urban development on the physical, chemical, and from their natural states to promote drainage and prevent biological characteristics of stream ecosystems in the South flooding (Klein, 1979). Commercial, residential, and industrial Platte River Basin. The 28 study streams are located along the development commonly involves soil disturbance, which can Front Range of the Rocky Mountains in the transition zone lead to increased movement of sediment to the stream, and the between the mountains and the plains. Study streams were removal of vegetation on the streambank, which can lead to chosen to represent (1) a wide range in the degree of urban loss of sheltered areas and stream canopy cover (Jacobson and development and (2) minimal natural variability due to factors others, 2001). The loss of stream canopy cover in turn can lead such as geology, elevation, and climate, which can also affect to greater daily changes in stream temperature (Sinokrot and stream ecosystems and therefore mask the effects of urban Stefan, 1993). development. Stream locations that are strongly affected by These changes in stream hydrology, water quality, physi- wastewater-treatment-plant discharge were not included in this cal habitat, and stream temperature in urbanizing areas can study. Because land use or population density alone often is have profound effects on aquatic communities of algae, inver- not a complete measure of urban development, the degree of tebrates, and fish. Periods of high streamflow can eliminate urban development in each drainage area was represented by some aquatic organisms, particularly in channelized streams U.S. Department of the Interior Fact Sheet 2006–3083 U.S. Geological Survey Printed on recycled paper August 2006 a multimetric urban intensity index derived from 16 variables, each stream for 4 to 6 weeks between May and June 2003 to including land use and land cover (such as developed area), concentrate trace levels of hydrophobic organic contaminants infrastructure (such as road density), and socioeconomic (such like polycyclic aromatic hydrocarbons (PAHs) and poly- as housing density) variables. Stream sites included in this chlorinated biphenyls (PCBs). The chemicals concentrated study covered an urban land-use gradient from minimal devel- in the SPMDs were analyzed for concentration and potential opment (urban intensity index = 0) to a very high degree of toxicity. Biological characteristics included algae and inver- development (urban intensity index = 100) in the drainage area tebrate communities, measured once during June 2003, and (fig. 1). fish communities, measured once during August 2003. In all, The objectives of the study were to (1) examine physical, 52 urban variables (including the urban intensity index), 153 chemical, and biological responses along the gradient of urban physical variables (including 53 habitat, 50 stream hydrology, development; (2) determine the major physical, chemical, and and 50 stream temperature variables), 225 chemical variables landscape variables affecting the structure of aquatic commu- (including 96 base-flow and 129 SPMD variables), and 75 nities; and (3) evaluate the relevance of the results to the man- biological variables (including 19 algae, 30 invertebrate, and agement of water resources in the South Platte River Basin. 26 fish variables) were included in the study. Physical characteristics included habitat, measured once between July and August 2003, and stream hydrology and Major Findings 106º 105º 104º Commonly observed effects of urban development on physical, chemical, and biological characteristics of stream Creek WY OM ING Bear ecosystems, such as increased flashiness (more rapid rise and NEBRAS KA fall of stream levels during and after storms), higher and more COLO RADO Lodg frequent peak flows, increased concentrations of chemicals, epol e C OMING reek and changes in aquatic communities, generally were not WY NEBRASKA observed in this study. w C ! Cro reek Lone tree C r e Cheyenne 41º e k COLORADO h Fork C ort a Physical Characteristics c N h e Cach Only one strong relation was found between the physical e La Poudre R iv er characteristics (stream hydrology, temperature, and habitat) Fort Collins ! and measures of urban development—decreases in stream Greeley temperatures occurred more rapidly as the percentage of ! iver ! B R atte high-intensity development near the stream increased (fig. 2). ig k Pl R on Loveland e South iv Thomps e e r r In urbanizing areas, inputs of ground water to the stream can C r aint e ! S C d in re l Fort decrease as more impervious surfaces are created. Because Vra e E k x Morgan o B ground-water temperatures generally fluctuate less than stream k e k e Boulder ee r r ! C temperatures, inputs of ground water can moderate stream 40º k B C o e u y ld eek r e er Cr r k D u e g C o i j e temperatures; thus, with less ground water entering the stream, i r B B C r stream temperatures may be falling more quickly. e v Clear a a Creek e w B Denver o None of the hydrology variables were related to any of ! i K Creek Bear the fish variables, and only a small number of the hydrology C variables were strongly related to a few of the invertebrate h e EXPLANATION No r rt r h y and algae variables—increased flashiness and higher peak Fo r Urban Intensity Index k P C l r u r e m e streamflows were related to changes in algae and invertebrate v 0–20 i e k R C communities associated with still-water habitats. During high e 21–40 t r t e a l e P k 41–60 streamflows, organisms living in still-water habitats can be h t u 61–80 o carried downstream in faster moving water. When periods of S 39º 81–100 high streamflow occur more often in flashier streams, it can be Base from U.S. Geological Survey digital data 1999 to 2004 difficult for organisms that prefer still-water habitats to persist. 1:24,000 to 1:100,000 Albers Equal Area projection Standard parallels 29º30'N and 49º30'N None of the habitat or stream-temperature variables were Central meridian 104º00'W Latitude of projection origin 23º00'N strongly related to any of the invertebrate, algae, or fish com- 0 50 100 MI LE S munity variables. 0 50 100 KILO METERS Chemical Characteristics Figure 1. Location and range of urban-intensity index values of Only two base-flow chemicals measured in August were the 28 study streams. strongly related to measures of urban development—sulfate concentrations increased in more rapidly growing areas and water temperature, measured on an hourly basis throughout the suspended-sediment concentrations decreased as housing age study. Chemical characteristics included nutrients, pesticides, increased (fig. 2). The increasing sulfate concentrations may major ions, and fecal bacteria, measured twice each during have been due to increased use of and runoff from impervi- base-flow conditions in June and August 2003, and hydropho- ous areas such as roads in urbanizing areas. The decreasing bic organics collected from semipermeable membrane devices sediment concentrations may have resulted from stabilization (SPMDs).