Scientific Investigations Report 2006-5136
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5136
In many basins in the western United States, on-going activities include enhancing fisheries, meeting rules implemented under the Endangered Species Act for resident and anadromous salmonid fish, and obtaining additional water for agriculture, population growth, and instream flows. The thermal regime of a riverine system and ground-water discharge to rivers is an issue for all these activities. The Yakima River Basin in eastern Washington, is typical of these basins (fig. 1), where the U.S. Geological Survey (USGS) is investigating ground-water/surface-water interactions and aquatic habitat for salmonids using the thermal regime. This report was prepared in cooperation with the Bureau of Reclamation, Yakama Nation, and Washington State Department of Ecology.
The purposes of this report are to document a method to thermally profile long (5-25 km), river reaches and to describe the importance of streamflow temperature and ground-water discharge, both of which are functionally related to a thermal profile in controlling aquatic ecosystems, and in particular fish. A thermal profile is a record of longitudinal measurements of stream temperature near the reach streambed. Measurements were made from a watercraft by towing in a Lagrangian framework one or two probes that measure and log temperature, depth, and conductivity at 1- to 3-second intervals. One probe is always used for measurements at the streambed and if a second probe is used, it measures near the surface. Spatial coordinates are logged with a Global Positioning System (GPS). Measurements can be referenced to a time, a position in space, or a distance from beginning of the profile.
The report first presents a background on the ecological importance of temperature and ground-water discharge in riverine systems and the methods typically used to measure these two environmental variables. The newly developed method to thermally profile reaches is then described. Last, an example of a thermal profile is presented and discussed to provide readers with an overview of a profile, including its information content, how a profile may be interpreted, and the reproducibility of the results using this method. To the extent possible, references and discussions are related back to the importance of temperature as a resource for fish, and in particular salmonids.
Processes that control the heat content of a parcel of surface water as it travels from headwaters to a river mouth are well documented. The processes include radiation, advection, conduction, evaporation, and convection (Anderson, 1954; Harbeck and others, 1959; Raphael, 1962; Messinger, 1963; Koberg, 1964; Edinger and Geyer, 1968; Edinger and others, 1968; Stevens and others, 1975; Jobson, 1977; Sinokrot and Stefan, 1993). Ground-water discharge represents a significant form of thermal advection in many river systems. Ground-water discharge (including water discharged from the hyporheic zone) (1) provides preferred thermal structure and habitat for different types of fishes at different life-history stages (Power and others, 1999), and (2) is an important abiotic variable of the aquatic ecosystem basic to the ecological function of riverine systems (Hynes, 1983; Stanford and Ward, 1993; Stanford and Simons, 1992; Brunke and Gonser, 1997). The ground- and surface-water interface is a unique ecotone and is similar to other ecotones known as some of the most productive habitats (Wetzel, 1990). Much of the year, streamflow is mostly baseflow—a product of ground-water discharge; therefore, water quality also is largely influenced by ground water. An overview of the current understanding of the interaction between ground water and surface water is presented in Winter and others (1998).
Temperature is one of the most important abiotic variables of the riverine system because it influences dissolved oxygen concentrations, photosynthesis, the metabolic rates of aquatic organisms, timing of life-history stages of many species, and the decomposition rates of organic material, which in turn, affect the spatial arrangement in the riverine system of many ecosystem components such as algal, invertebrate, and fish communities. Therefore, the bioenergetics of riverine ecosystems ultimately is determined by the thermal regime. The presence of diversity and structure in the temperature regime leads to increased biodiversity (Magnuson and others, 1979), including biodiversity of fish (Brett, 1956; Beschta and others, 1987), insects (Vannote and Sweeney, 1980), and macrophytes (Haslam, 1978; White and others, 1987). Diversity represents long temporal variations from expected heating trends (overall thermal profile shape—longitudinal gradient) and structure represents the short spatial-temporal variations (the spikes in the thermal profile). Diversity and structure display unique spatial patterns that are functionally related to ground-water discharge. In turn, the variations in the thermal diversity and structure increase with basin size and attendant variations in climatic regimes and landscape characteristics—an upland 5 km2 headwater basin has much less variation than a large 12,000 km2 basin.
Diversity in a basin’s thermal regime represents the riverine system’s temperature template or longitudinal gradient, which is consistent with an environmental gradient. Temperature essentially defines a physical habitat template (Southwood, 1977; Poff and Ward, 1990) that explicitly includes temporal variability and provides for the overall biological community template—including the different life stages and life history patterns of salmonids. The template leads to a logical progression of the longitudinal gradient of fish assemblages. Small scale temperature variations (structure) represent cooling patches (possible refugia) or heating (avoidance areas) overlaid on the basin-wide template, and reflect the localized lateral and vertical connections measured in both natural and modified river systems (Hynes, 1983; Stanford and Ward, 1993) that salmonids use or avoid (Power and others, 1999; Rieman and Dunham, 2000). These ground-water discharge zones provide refugia, the preferred salmonid habitat during summer when river temperatures are warm and during winter in colder regions when rivers may freeze. Salmonids seek out and take advantage of this habitat. The longitudinal gradient, overlaid with the distribution of patches, composes a continuum from the headwaters to the mouth, along which habitat and species are arranged (Vannote and others, 1980).
The thermal regime at salmon redds (gravel spawning nests) also is important for incubation (Combs and Burrows, 1957; Combs, 1965; Alderdice and Velsen, 1978). The thermal regime and ground-water discharge effects egg survival for salmonid species (Sowden and Power, 1985; Woessner and Brick, 1992: Curry and others, 1995) including wild bull, rainbow, steelhead, and kokanne trout in the Yakima River Basin. Ground-water discharge areas also appear to be the preferred winter habitat for trout (Brown and Mackay, 1995), and temperature habitat limitations may affect behavior (Gregory and Griffith, 1996). Power and others (1999) identified winter as critical time (1) when overwintering fish mortalities may be high and (2) in the establishment of basic stock densities based on temperature habitat availability.
In large diverse basins with an extensive riverine system, documenting the thermal regime is difficult. Methods typically used to document the regime are continuous measurements at fixed stations, synoptic manual measurements, and multi-spectral imaging. Fixed station and synoptic data measure only heat content of a particular water parcel, so information on the spatial structure of the thermal regime, which describes a habitat template, must be interpolated. However, accurate interpolation requires additional measurements (longitudinal, transverse, and advectional) to quantify thermal variation in a stream. Using multi-spectral techniques such as forward-looking infrared radiometer (FLIR) can yield the thermal regime of a riverine system, but only for one period, and these methods are costly and synoptic. How the regime varies temporally is unknown. Moreover, FLIR measures surface radiance and does not document vertical structure. As a result, excluding features such as springbrooks, ground-water discharge cannot be precisely located with FLIR until manifested at the water surface.
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