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Scientific Investigations Report 2006-5101-A

Effects of Urbanization of Stream Ecosystems in the South Platte River Basin, Colorado and Wyoming

By Lori A. Sprague, Robert E. Zuellig, and Jean A. Dupree

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The citation for this report, in USGS format, is as follows:
Sprague, L.A., Zuellig, R.E., and Dupree, J.A., 2006, Effects of urbanization on stream ecosystems in the South Platte River Basin, Colorado and Wyoming, chap. A of Effects of urbanization on stream ecosystems in six metropolitan areas of the United States: U.S. Geological Survey Scientific Investigations Report 2006-5101-A, 139 p.


This report describes the effects of urbanization on physical, chemical, and biological characteristics of stream ecosystems in 28 basins along an urban land-use gradient in the South Platte River Basin, Colorado and Wyoming, from 2002 through 2003. Study basins were chosen to minimize natural variability among basins due to factors such as geology, elevation, and climate and to maximize coverage of different stages of urban development among basins. Because land use or population density alone often are not a complete measure of urbanization, land use, land cover, infrastructure, and socioeconomic variables were integrated in a multimetric urban intensity index to represent the degree of urban development in each study basin. Physical characteristics studied included stream hydrology, stream temperature, and habitat; chemical characteristics studied included nutrients, pesticides, suspended sediment, sulfate, chloride, and fecal bacteria concentrations; and biological characteristics studied included algae, fish, and invertebrate communities. Semipermeable membrane devices (SPMDs), passive samplers that concentrate trace levels of hydrophobic organic contaminants like polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), also were used. The objectives of the study were to (1) examine physical, chemical, and biological responses along the gradient of urbanization; (2) determine the major physical, chemical, and landscape variables affecting the structure of aquatic communities; and (3) evaluate the relevance of the results to the management of water resources in the South Platte River Basin.

Commonly observed effects of urbanization on instream physical, chemical, and biological characteristics, such as increased flashiness, higher magnitude and more frequent peak flows, increased concentrations of chemicals, and changes in aquatic community structure, generally were not observed in this study. None of the hydrologic, temperature, habitat, or chemical variables were correlated strongly (Spearman’s rho greater than or equal to 0.7) with urban intensity, with the exception of some of the SPMD-based toxicity and chemical variables. SPMD-based measures of potential toxicity and PAH concentrations were positively correlated with urban intensity. The PAH concentrations also were positively correlated with measures of road density and negatively correlated with distance to the nearest road, indicating that automobile exhaust is a major source of these compounds in the study area. This source may be localized enough that the transport of PAHs would be minimally affected by water-management practices such as diversion or storage upstream. In contrast, the predominant sources of nutrients, bacteria, suspended sediment, sulfate, chloride, and pesticides may be more dispersed throughout the drainage area and, therefore, their transport to downstream sites may be subject to greater disruption by water regulation. Although no direct link was found between most water-chemistry characteristics and urbanization, invertebrate, algae, and fish-community characteristics were strongly associated with nutrients, pesticides, sulfate, chloride, and suspended sediment.

None of the biological community variables were strongly correlated with the urban intensity index. Algal biomass predominantly was associated with total nitrogen concentrations, nitrite-plus-nitrate concentrations, and the duration of high flows. Fish communities predominantly were associated with housing age, the percentage of suspended sediment finer than 0.063 millimeters and chloride concentrations. Invertebrate communities predominantly were associated with the frequency of rising and falling flow events, the duration of high flows, total nitrogen concentrations, nitrite-plus-nitrate concentrations, and total herbicide concentrations.

Historical records indicate that aquatic communities in the region may have been altered prior to any substantial urban development by early agricultural and water-management practices. Present-day aquatic communities are composed primarily of tolerant species even in areas of minimal urban development; when development does occur, the communities already may be resistant to disturbance. In addition to the effects of historical stressors on aquatic community structure, it is possible that current water-management practices in the study basins are having an effect. In the absence of natural, unaltered hydrologic conditions, more sensitive taxa may be unable to recolonize urban streams. The movement and storage of water also may lead to a disconnect between the land surface and streams, resulting in instream physical, chemical, and biological characteristics that, to some degree, are independent of land-cover characteristics.

Table of Contents



Purpose and Scope

Study Area



Site Selection

Variability in Natural Landscape Features

Gradient in the Degree of Urbanization

Suitability of Local Site Conditions

Data Collection

Physical Characteristics

Stream Hydrology

Stream Temperature


Chemical Characteristics

Biological Characteristics

Algae Communities

Fish Communities

Invertebrate Communities

Data Analysis

GIS Variables

Calculation of the Final Urban Intensity Index

Additional GIS Variables

Physical Characteristics

Stream Hydrology

Stream Temperature


Chemical Characteristics

Seasonal Variability

Spatial Variability

Biological Characteristics

Resolving Ambiguities

Community Summarization

Effects of Urbanization on Stream Ecosystems

Response of Physical Characteristics

Stream Hydrology

Stream Temperature


Response of Chemical Characteristics

Seasonal Characteristics

Spatial Characteristics

Water Chemistry

SPMD-Based Toxicity and Chemistry

Comparison of Water Chemistry and SPMD-Based Toxicity and Chemistry

Response of Biological Characteristics

Algal communities

Algal Response to Urbanization

Algal Response to Hydrology and Habitat

Algal Response to Water Chemistry

Fish Communities

Fish Response to Urbanization

Fish Response to Hydrology and Habitat

Fish Response to Water Chemistry

Invertebrate Communities

Invertebrate Response to Urbanization

Invertebrate Response to Hydrology and Habitat

Invertebrate Response to Water Chemistry

Major Findings from Response Characteristics

Summary and Conclusions

References Cited

Appendix 1. GIS Variables

Appendix 2. Physical Variables

Appendix 3. Chemical Variables

Appendix 4. Data from Semipermeable Membrane Devices

Appendix 5. Biological Variables

Appendix 6. Scatterplots of Variables Presented in Report Tables

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