Water quality varies greatly in nature. We have added to the variability by knowingly or unintentionally modifying the natural state of our waters. When such activities result in the reduction or elimination of the species present in a natural water body, we think of it as being polluted.
How many decimal places?
When I took classes out to lakes and streams to sample environmental variables and organisms present, students were often concerned with precision of their measurements. There are some things where that is important, but temperature, pH, and oxygen content of the shallows of lakes can often show great cycling during a day. Sun can (1) warm undisturbed surface waters rapidly, (2) stimulate high levels of photosynthesis causing (3) dissolved oxygen to rise. At the same time alkalinity can change and carbonate deposition can occur. So great precision is almost meaningless and results are variable from day to day depending on temperature and cloud cover. For the same reason, small differences between different bodies of similar water are inadequate to be the sole reason for their characterization. Salinity, phosphate, and toxic substances are things where greater precision is important.
There are plenty of other places where highest possible precision in measurements are important in research as well as in medical and other studies.
Temperature
Most species have an optimal water temperature they will occupy if it is available. In Michigan, lumbering reduced the shading of streams and made temperatures go to higher extremes in the summer so trout and other cool water species could not survive. Erosion also increased and sometimes covered stream bottoms, changing the bottom fauna serving as food.
Mountain streams often erode rocks and other material during periods of snow melt or heavy rains and provide a heavy load of sediment in the water. The picture below shows the clear waters from Lake Geneva on one side of the Rhone River while sediment laden water of the Arve River coming from the Alps is strikingly obvious on the opposite side.
Incomplete Mixing of the Arve and Rhone Rivers, 1952
Your municipal water supply intake siting should consider all possible variables to minimize problems. A small outlet from a local lake had noticeable difference in surface water temperature and salinity within just a few inches. The lake (Asylum Lake in Kalamazoo) has an elevated salt content in deeper water from deicing salt runoff from roads. So much so that it does not mix completely with the upper water. As the ice cover melted during a windless spring day, the surface layer did not mix much with the deeper water because flow was gentle and non-turbulent to the outlet. The phenomenon* is not of much consequence but does illustrate the possibilities of unusual effects.
Estuaries
Tidal effects interacting with the mixing of sea and freshwater streams produce some interesting anomalies. Large freshwater streams in the Northern Hemisphere tend to support freshwater organisms farther downstream on the right side as moving water is deflected to the right (but to the left in Southern Hemisphere) and salt water and marine species reach farther upstream on the opposite side for the same reason (called the Coriolis Effect).
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*I submitted the observation to a water pollution journal about thirty years ago hoping they would publish it as a very short note. But they wanted more than the one instance to justify the publication. I did check other comparable locations and found it also occurred during the spring ice melt. I thought it should alert pollution biologists to the need for more than one sample in streams under some circumstances to justify estimates of pollution loads below dams etc.
Joseph G. Engemann Kalamazoo, Michigan March 16, 2016
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