Selected Water Quality Processes in Rivers and Streams - Hydro-Environmental Analysis: Freshwater Environments - page 147

T (°F)

40 50 60 70 80 90 100

16

Sea level

12

1.6 km

8

3.2 km

4

4.8 km (3 miles)

0 0

10 20

T (°C)

30 40

FIGURE 5.22 Variations in dissolved oxygen saturation concentrations ( O s ) as a function of temperature.

There are a number of well-known methods to estimate saturation concentrations, and one com-
monly used formulation is published by the American Public Health Association (APHA 2005).
Agencies such as the U.S. Geological Survey (USGS) also provide saturation calculation tables on
their websites (DO saturation tables: http://water.usgs.gov/software/DOTABLES/).

5.5.2 r eaeratIon
Reaeration refers to the exchange of oxygen between water and the atmosphere. The net rate of
reaeration depends on the factors inluencing the exchange and the oxygen gradient between water
and the atmosphere, as expressed by the saturation concentration discussed in Section 5.5.1. If the
water concentration is at saturation, there is no gradient and the net exchange is zero. The greater
the gradient is, the greater the next rate of exchange will be.
Since the exchange rate occurs at the water surface, any forcing, such as turbulence introduced
by currents or wind that “stirs” the water, will increase the rate of exchange. For example, the rate of
exchange in a stagnant reach of river would be slow, even if the gradient were large. However, wind
and wave action would enhance the transfer. Similarly, the rate of reaeration in a stream or river is
directly proportional to the current velocity, and inversely proportional to the depth.
Reaeration can also be induced by turbulence in rifle areas and waterfalls in a stream or river
or by lows over structures such as weirs (Figures 5.23 and 5.24). In some cases, structures are
designed for the speciic purpose of increasing DO concentrations.

FIGURE 5.23 Reaeration over waterfall. (Photograph by J.L. Martin.)