Figure 1.3.2 Idealized reservoir tailwater depicting morphometric features (e.g. riffles
and pools), downstream users, material transport/deposition scenarios, and flow related
velocities. The development of a backwater area and sediment deposition is depicted downstream from
the confluence of the secondary tributary. Further downstream a low-head dam has been constructed
forming a pool for a water intake and resulting in decreased velocity and increased sediment deposition.
The low-head dam also provides aeration, mixing, increased velocity, and increased sediment transport
as water passes over the dam. A point discharge downstream from the low-head dam is also shown
indicating that specific sources of constituents also exist in tailwaters.
Physical changes due to impoundment, hydrology, and morphology are primarily reflected by a
change in velocity, temperature, amount and distribution of suspended solids (and hence, available
light), and concentrations of dissolved gases. Chemical changes are most often the result of water
quality conditions in the upstream impoundment, biological processes in the tailwater, and
physicochemical changes in the water column (including air/water and water/channel bottom interfaces).
Examples of chemical changes include the increase in concentrations of nutrients and reduced elements
from anoxic hypolimnia, decreases in dissolved oxygen concentrations with distance due to biological
oxygen demand of organic matter from the upstream impoundment, precipitation reactions associated
with oxidation of reduced metals, degassing, and sorption/desorption reactions on substrates.