OCR Text |
Show Major Findings 19 occasional adverse biological effects are expected. Adverse biological effects are generally defined as effects that are considered to produce a negative response in an organism, such as reduced reproductive success, reduction in growth, or death. Increased urban land use has altered invertebrate and algal communities Researchers have shown that as urbanization increases in a watershed, streams typically show a decline in water quality and condition of the physical habitat, which in turn leads to a decline in the quality of biological communities in the streams. ( For a review of literature on effects of urban land use, see Paul and Meyer, 2001.) Effects of urbanization on streams along the Wasatch Mountains were examined using a special " urban gradient design" ( see inset). Differences in the physical, chemical, and biological characteristics of 30 sites along a gradient of urbanization were compared with the level of urban intensity in each watershed to see if urbanization was having a measurable effect on these characteristics. The area included in the urban- gradient assessment has strong natural gradients, and the hydrology and chemistry of the streams are affected to a large degree by the movement of water across watershed boundaries to meet human water- use needs. The urban- gradient assessment showed that water quality of the streams declined with increasing levels of urban intensity. Specifically, increasing concentrations of nutrients ( nitrogen and phosphorus), warmer water temperatures, a greater number of pesticides, and increasing specific conductance were associated with increasing urban intensity ( fig. 32). Specific conductance is a measure of dissolved ions in the water, and increasing values most likely indicate increased runoff associated with increasing urban intensity. Urban runoff commonly contains contaminants that are harmful to biological communities. An increase in dissolved an Gradient Study The Great Salt Lake Basins Study Unit was one of several NAWQA Study Units that participated in an urban land- use gradient study. This special study was designed to examine the effects of urbanization on streams by sampling physical, chemical, and biological characteristics of 30 sites along a gradient of low to high levels of urbanization ( fig. 31) and to compare how urbanization affects streams in different parts of the country. An integrated index of " urban intensity" was used to define urban effects at each site ( McMahon and Cuffney, 2000). Each watershed was ranked in urban intensity ( ranging from 0 to 100) on the basis of 13 measures of urbanization, including measures of land cover ( for example, percentage of land surface covered with vegetation), urban infrastructure ( for example, the density of roads), and socio- economic characteristics ( for example, the density and type of housing, population density). For reference, an urban index of 50 represents approximately 30 percent land cover that is urbanized and an average road density of approximately 17 kilometers of roads per square kilometer area. A low rating on the urban index means there is very little urban development in the watershed and the land is mostly forested. A high rating on the urban index represents a watershed with a mixture of higher density housing and( or) commercial space. Along the western front of the Wasatch Mountains, the intensity of urbanization tends also to follow the elevation gradient, with the most intensely urbanized areas at the valley bottom. The characteristics of streams, be they physical, chemical, or biological, also change along the elevational gradient, regardless of differences in urbanization. Because the objective was to measure changes due to urbanization, the study sites were limited to only those on the benchlands of the Wasatch Mountains, and, as a result, the study did not sample the high intensity urban areas at the valley bottoms. ^ V Ogden . •\ Weber Siv, "!^ Creek O? Inset C 1* 3^ EXPLANATION Hi Urban area • Urban land- use gradient site "^ Figure 31. Thirty sites were sampled for fish, macroinvertebrates, algae, water chemistry, and physical habitat along a gradient of urban intensity. |