Water Temperature Influences in Rivers and Streams. <<<...return
The temperatures of rivers and streams vary much more rapidly than those of lakes. Depending upon size and origin of streams, their seasonal and diurnal temperatures follow atmospheric temperatures more closely than do those of lakes. The source of the stream water and, in some instances, the nature of the drainage pattern determines the thermal properties of a stream. For instance, water temperature varies along the lengths of the valleys.
Large rivers and streams at some distance from their sources are usually in a proportional equilibrium with mean monthly air temperature.
The temperature of headwater streams is variable, but as the downstream water volume increases and becomes more constant, the range of temperature variation decreases. With respect to stream size and water temperature fluctuations, we can conclude that the smaller the stream, the greater the temperature variation and the more rapid the response to environmental fluctuations of ambient air temperatures.
Superimposed on the annual relationship of atmospheric temperature with stream size are often special features. For instance, during spring, snowmelt water may keep the temperature below that of the air for quite some time. Also, sunshine after a heavy rain has been observed to raise water temperatures presumably because water from the warm soil was continuing to flow into the stream.
In summer, the headwaters are relatively cool either because they contain spring water is at the annual mean soil temperature or because they come from high elevations and are only slowly warmed by the air, the sun and conduction from the ground.
In winter the reverse may apply to spring fed streams that start relatively warm and become cooler. Also, during cold weather ice may form on the waters surface although it does so less readily on running rather than still water. Once ice has been formed, snow can accumulate on it and together they form an excellent insulator against further heat loss. Very probably, however, even where the water is frozen down to the streambed, free water remains between and underneath the stones.
Diurnal cycles are changes over the 24 hour day that add to the seasonal temperature changes. Diurnal temperature variations at a given point in a stream may be related to two major sets of factors:
(1) conditions at the sample location, and
(2) conditions upstream from the sample location.
Under point 1, we should consider velocity and discharge, season and hour, and the daily range of fluctuations of air temperatures at that point.
Factors pertaining to point 2 include the nature of the upstream environment, substrates, atmospheric conditions, temperatures, distance and time of flow from critical upstream situations.
The range of daily variation of water temperature is largest when there is the greatest differential between mean diurnal air temperature and mean water temperature.
With increased volume and turbidity, the range of variations is reduced. These may be about 6° C. in small streams in the summertime, with lower values being observed in larger rivers. Also, in small streams, the deeper the water the less is the daily variation. Maxima usually occur in the afternoon and minima in the latter half of the night. This is caused primarily by the radiation into and out of the water. It will be appreciated therefore that the shallow streams a few meters wide, especially if they are not shaded from the sun, are particularly subject to short-term or diurnal variations. Furthermore, these fluctuations exist throughout the year, but are minimized beneath the ice cover.
Because of turbulence and the shallow nature of most streams, thermal stratification is not generally an attribute of streams. When stream waters do stratify, the process usually takes place in pools along the stream course. This may be caused by sunshine on the shallows, inflowing groundwater or the water from a tributary hugging the bank on which it had entered.