Depositional Zone
The depositional zone refers to the inner bank of a stream where velocity is at a minimum (see Figure 4-6). The slower velocities allow for the deposition of suspended sediment and bed materials (gravel, pebbles), which form bars. These bars often support emergent aquatic vegetation.
A pool is relatively deep and wide with slow-moving water compared to riffle, run, or glide areas. Pools often contain large eddies with widely varying directions of flow compared to riffles, glides, and runs, where flow is nearly all downstream. Eddies are currents that move in a direction other than downstream, usually in a circular motion. Pool areas support fish, aquatic invertebrates, and aquatic plants (see Figure 4-6).
Sediment in most pooled areas of streams and rivers is composed of sand, silt, clay, and organic matter, compared to the coarser sediment of riffles, runs, and glides. Reduced velocity allows suspended materials to settle to the bottom. The slower-moving water supports organisms similar to those found in lakes and pond systems (dragonflies, damselflies, water striders). For descriptions and illustrations of common freshwater macroinvertebrates, see Chapter 6.