Scientific Investigations Report 2006-5136
U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2006-5136
The thermal profile method developed for long river reaches proved to be robust, reliable, and reproducible and can be applied while drifting in a Lagrangian framework, wading, and by using motorized watercraft. Motorized watercraft are best used in quiescent waters such as lakes or boat accessible, non-braided stream reaches. The CTD-Diver™ probe and its container functioned excellently under adverse conditions—typified by large temperature variations (reach temperatures varied from about 2 to 24°C), and continual contact and impact with the streambed. Under both 1- and 3-second sample rates, the CTD-Diver™ probe’s storage was not exceeded and the response of multiple sensors was good. Streaming of spatial coordinates using a Global Positioning System (GPS) connected to a laptop computer also functioned well, as did the self-contained GPS. Considering that the terrain varied greatly over the study reaches and included canyon type areas and heavily forested stream banks in more headwater areas, the loss of GPS reception did not occur as often as may be expected. Reception loss occurred less than 5 percent of the time; however, it can be problematic. The method’s advantages and cost-effectiveness outweighs its limitations.
Profiles for the example reach (1) display much intra-profile diversity and structure; (2) indicate that the diversity and structure measurements are reproducible; and (3) identify ground-water discharge areas. Minimum ground-water discharge values were estimated for a broad area. These values were conservatively estimated to range from about 6 to 9 percent of measured upstream discharge, and under the assumption of equilibrium temperature, they are within the range of error in the discharge value from the streamflow measuring station at the upstream end of the reach. The method is able to locate ground-water discharge areas that may not be identifiable from discharge measurements. These ground-water discharge areas were located using deviations from the diurnal heating. In the thermal profile, broad discharge areas are typified by stabilization, cooling, or declining rate of change in temperature increases. Local discharge is exhibited as structure (short temporal variations) contained in the thermal profile. Some local discharge areas were not only small, but were identified based on minor water temperature changes while drifting at high velocities (1.2–1.8 meters per second).
Thermal processes leading to end-member temperatures of a reach could not have been identified using available techniques. Realistic thermal modeling of these reaches would be time-consuming and would not provide details of the thermal habitat information contained in the profiles.
For habitat assessment, thermal profiles delineate the thermal habitat at a small scale. When combined with detailed pool-riffle-run structure documented in the depth data, a clearer picture of habitat for salmonids can be obtained. Profiling of many reaches and inclusion of fish survey data can potentially lead to an improved understanding of the relation between the different life-history stages of salmonids and the thermal-depth habitat in a basin.
Although this method does not supplant other methods, it is a cost-effective way to enhance analysis and use of data gathered using other techniques. Thermal-depth profiling of headwater and first to second order streams was not tested. In many areas in the western United States, these parts of streams represent the only remaining patches of suitable habitat for some salmonids (Rieman and Dunham, 2000). Determining whether this method can be applied in these areas would be beneficial, because terrain may disallow GPS reception. This leads to the question of the value of a thermal-depth profile for a reach that is not tied in with spatial coordinates, especially because temperature is thought to be a limiting factor for most life-history stages of salmonids in many river basins. Without spatial coordinates, a thermal profile completed between two fixed stations may well provide valuable information on the response of a water parcel as it moves downstream in these environments. Thus, providing a better estimate of the value of the fixed station data relative to processes occurring along a reach.
A thermal profile provides valuable information on spatial and temporal variations in the thermal regime and habitat, indicates ground-water discharge areas, and identifies areas for more detailed study. Thermal profiles display diversity and structure that can not be captured by fixed station data.
U.S.
Department of the Interior | U.S. Geological
Survey
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