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 While often considered to adversely impact the downstream habitat, positive effects of dam
construction can also occur and will be included in the following discussion. In a broader context,
Palmer and O'Keefe (1990) suggest that reservoirs receiving agricultural and urban runoff can improve
conditions in the downstream area, emphasizing the relationships to the catchment area. The objective
of this section is to provide an overview of water quality processes in reservoir tailwaters and their
relationships to project operations. While this discussion focuses on processes downstream from
selected dams, the logical expansion to the basin or river level with multiple impoundments has been
initiated by Ward and Stanford (1983) and referred to as the Serial Discontinuity Concept which is
modeled after the River Continuum Concept of Vannote et al. (1980). More detailed information is
available in the literature and excellent references include Hynes (1970) and Petts (1984).
1.3.2 HYDROLOGY/MATERIAL TRANSPORT
The movement of water or hydrology in a reservoir tailwater greatly affects the transport of
material. Flow patterns vary from laminar (parallel flow at a constant low speed) to turbulent
(characterized by irregularity of flow moving in different directions and speeds) flow as velocity
increases (Figure 1.3.3). Turbulent flow is an erratic and mixing progression of water and describes
most of a stream's flow pattern. As the slope of the river channel increases, accelerating forces are
checked by turbulence and channel roughness which induces turbulence, and velocity is limited. The
boundary between the smoothness of laminar flow and the eddying
Figure 1.3.3 General diagram of laminar and turbulent flow
sinuosity of turbulent flow is referred to as the Reynold's number (NR) which is dimensionless. In the
stream channel it is the value derived from dividing the product of the mean grain diameter (), the
specific discharge (q), and the fluid density (d) by the dynamic viscosity () and is effected by
temperature affects on density and viscosity.
1.3-4
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