7922 - Laserfiche WebLink

Home Browse Search

Given a constant supply of conservative chemicals, the concentration decreases with increased discharge and vice versa. Nonconservative constituents are those materials which, because of bio- chemical reactions, change in concentration over time. Typical examples of nonconservative constituents are oxidizable organic material, ammonia, and dissolved oxygen. Nonconservative chemical concentrations are related to streamflow by numerous mechanisms. For example, consider the factors that affect dissolved oxygen concentration. The saturation concentration of dissolved oxygen and the decomposition rate of organic materials are both governed by temperature, and temperature is related to streamflow in several ways. First, there is a greater volume of water to be heated by the sun with more discharge. Second, velocity determines the travel time for a body of water, and the longer it takes for the water to travel through a system, the longer it is exposed to thermal gain (or loss). Third, the larger the ratio between the width and depth of flow, the greater the proportion of the water volume exposed to sunlight. This ratio is often increased with reduced discharge. Reduced discharge generally results in higher water temperatures during the summer, at least until a thermal equilibrium is reached with ambient atmospheric conditions. As temperature increases, the saturation concentration for dissolved oxygen decreases and the rate of oxidation of organic material increases. The dissolved oxygen concentration is also a function of the concentra- tion of oxygen-demanding organic material. Therefore, dilution of organic matter by the total discharge is an important determinant of the dissolved oxygen concentration. In addition, water velocity plays a major role in the dispersion of oxygen-demanding material, both locally and longitudinally. Insufficient velocity allows settling of larger organic particles, which can lead to the formation of sludge deposits. Higher water velocity disperses the organic material over a larger longitudinal distance. This means that a larger area of stream is involved in the oxidation process at higher flows, reducing the demand on reaches closest to the source of the oxidizing material. Finally, the rate of mechanical reaeration is a function of depth and velocity. Temperature alone may have significant effects on a community, in addition to its role as a driving variable in the dissolved oxygen equation. The most obvious effects of temperature are on survival and growth of aquatic organisms. Less obvious is the effect of temperature on the timing of the life history stages of a species or species phenology. The metabolic rate of all coldblooded animals is directly related to temperature. A disruption in the thermal regime of a river may make certain stream reaches uninhabitable for some species, but not for others. In some cases, the temperature may be so high that a reach will be totally uninhabit- able or so low that growth is impaired. Alternately, a source of very warm or very cold water may block the migration of a species into upstream areas where the habitat is satisfactory. The longitudinal distribution of a species implies at least some avoidance of areas where temperature may affect survival. However, suboptimal temperatures, even though nonlethal, may significantly reduce fish production. The determination of the length of stream having suitable water quality and temperature is illustrated in the top half of Figure 4. Several pathway 12