Flows and Transport in Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 106

250

200

150

100

50

0

1935 1945 1955 1965

Water year

1975 1985 1995 2005

FIGURE 4.24 Minimum streamlows in the Big Sunlower River. (From Hirsch, USGS science in sup-
port of water resources management, Presented at Mississippi Water Resources Conference, Jackson, MS,
20 0 6.)

(longitudinal) hydraulic models, with the assumption that lateral and vertical variations are
relatively small.
Since lows are the total lux of water through a section (the product of velocity and area), lows
only provide an estimation of that average longitudinal transport. So, for example, the low divided
by the area provides an estimate of the average velocity. Some distance down the river divided by
the velocity provides an estimate of the average travel time.
But, in the estimation of a low, it is necessary to measure velocities over the width and depth
of a channel because they do vary. That is, the velocity varies with the depth and width. Variations
may also occur longitudinally, since natural rivers do not occur in straight prismatic channels, as
illustrated in Figure 4.25.
One consequence is that materials injected into a river, such as a dye, will move at different veloci-
ties depending on how and where they were injected. For example, if, initially, the dye were com-
pletely mixed over the cross section, then downstream, the dye located near the center of the river,

Fast

0 1

Meters

Slow

Medium
Fast

Slow

Cross section of a river showing the general
pattern of current velocity

Example of current velocity patterns in a river

FIGURE 4.25 Variations in current velocities in natural rivers. (From Hebert, P.D. (ed.), Canada's Aquatic
Environments , CyberNatural Software, University of Guelph, 2002. Available at http://www.aquatic.uoguelph.
ca/. With permission from the Biodiversity Institute of Ontario.)