Chapter 9: Coldwater Fish in Rivers
R. Allen Curry, Robert M. Hughes, Mark E. McMaster, and David J. Zafft
Rivers are large natural streams or reservoir tailwaters most effectively and safely sampled from a boat or raft. They may have wadeable riffles, bars, or shorelines, but because of current velocities or depths of pools and runs, they are generally unwadeable over segments of 10–100 mean wetted channel widths. In most instances, coldwater rivers are occupied by trout and salmon, sculpins, or other coldwater taxa. Midsummer temperatures in the main stems of coldwater rivers usually do not exceed 20°C, although coldwater habitats (e.g., groundwater springs or tributary inputs) may support coldwater species during periods when temperatures are greater than 20°C. The water in coldwater rivers can vary in color (0.5–4.0 mg dissolved organic carbon per liter), turbidity (0–10 nephelometric turbidity units), and conductivity (25–1,000 μS/cm). Nutrient and chloride concentrations are usually low. Exceptions can occur in watersheds where human activities contribute to the river flows (e.g., wastewater discharges, agricultural runoff, pulp and paper effluents, or mining effluents). Dissolved oxygen concentrations are generally greater than 5 mg/L and more than 80% saturation. Coldwater rivers are typically less productive than warmwater rivers because of lower temperatures and nutrient concentrations, but cold reservoir tailwaters are often more productive than unregulated coldwater rivers because of nutrients and production from the reservoirs.
When sampling coldwater rivers, it is important to be aware of other activities that may conflict with sampling (e.g., recreational anglers, boaters, canoeists, commercial and native fisheries, transportation vessels, dams, water diversions, or discharge sites). Large vessels, dams, diversions, and discharges create serious hazards for the unwary.
Coldwater fish assemblages are typically dominated by salmonids (ciscoes, whitefishes, salmons, trouts, chars, inconnus, or Arctic grayling). In addition, burbot, sculpins, minnows, and suckers are often abundant. Lampreys, sturgeons, eels, herrings, mooneyes, pikes, trout-perches, and perches may be present.