ABSTRACT
The physical and water quality characteristics of rivers and streams vary widely from their headwaters to their terminus, over their cross sections and over time, as described in Chapters 2 through 5. These physical and water quality variations also directly impact the presence and distribution of the biota of streams and rivers. Organisms may have specific ranges of depths, velocities, substrate types, temperatures, dissolved oxygen, and other physical and chemical factors that they can tolerate. The range of conditions from a preferred or optimal condition that can be tolerated may be broad or narrow depending on the organisms and, in many cases, depending on the particular life stage of the organisms. For example, organisms found in headwater or low-order streams may be less tolerant to high temperatures and temperature variations, than those found in high-order streams. In low-order, high-energy streams, the velocity may be sufficient to armor the bed, so the majority of the substrate may consist of gravel, rocks, and boulders. Some organisms may be more tolerant of higher velocities and resist being washed away by attaching to this coarse substrate, or by living in interstitial waters.
