Chapter 20: Warmwater Streams
Daniel C. Dauwalter, William L. Fisher, and Frank J. Rahel
Warmwater streams are those streams and rivers with warm temperatures and support diverse fish assemblages including populations of basses, sunfishes, and catfishes. Warmwater streams are distinguished from coldwater streams because they lack salmonid populations, typically occur at lower elevations, and have cool to warm water in summer, medium to high streamflows, clear to turbid water, diverse substrates, and low gradients (Winger 1981). Warmwater streams occur throughout the United Mexican States (Mexico), the USA, and central Canada, except in mountainous regions in the west and north. Fishing in warmwater streams occurs in the entire USA except Alaska and is the predominant type of fishing in over half of those states (Funk 1970). Not surprisingly, the criteria for classifying a stream as warmwater differ among individuals and management agencies. One criterion is the presence of trout: if trout are present, then the stream is considered to be a coldwater stream; if they are absent, then it is classified as a warmwater stream. Other criteria use water temperature statistics to classify streams (e.g., instantaneous maximum, daily mean, or monthly mean); an average daily summer water temperature 20°C or more is often used as general rule to define warmwater streams (Winger 1981). Adding to the confusion is the fact that coldwater streams can become warmwater streams when anthropogenic disturbances increase stream temperatures, and classifications can be based on either current conditions or potential conditions in the absence of disturbance. Regardless, criteria for classifying streams as coldwater or warmwater are based on management goals.
Because warmwater streams occur throughout North America, their physical and chemical characteristics vary in relation to their environmental setting. Most of the USA has a temperate climate with moderate air temperatures and rainfall amounts, but warmwater streams also occur in the hot, dry climate of Mexico and the southwestern USA; the cool, moist, continental climate of the northern USA and Canada; and the warm, wet, subtropical climate of southern Florida.