Is the water dark green and full of algae, with no sign of fish or other types of water animals?

Algae may be using up all the oxygen in the water and choking off other forms of life.

Oil and Petroleum Products

Oil pollution is commonly perceived as being a problem associated mainly with the marine environment, but in fact oil and related substances account for about one-quarter of reported pollution incidents of fresh waters in Britain; and probably the pattern is not much different in other parts of the world. Also, about one-quarter of all the oil released to the seas by human activity is estimated to enter via rivers. The sources of oil pollution are the usual ones - illegal, negligent or accidental discharges, plant failures and so on - but to these must be added inputs from roads and railways, particularly in the case of traffic accidents, and discharges associated with oil extraction from inland oilfields and processing plants.

The effects of oil discharges vary enormously, because the characteristics of the receiving waters and the various types of oil are themselves very variable. Fortunately many small discharges have relatively trivial effects, but substantial discharges can have severe impacts. Generally, light oils are relatively volatile and disperse quickly, though they are extremely toxic and can cause severe local damage. Fast-flowing waters often recover rapidly, over a few weeks or months, but lentic waters are much more susceptible to long-term damage. Heavy oils are less toxic, but can have marked physical effects in the substratum and the banksides, which of course will be reflected in changes to the biological community. Heavy oils may remain in situ for long periods; they do, over time, decompose biologically and chemically, but in so doing impose a high BOD. Frequently, the effects of oil pollution are not dissimilar from those of heavy organic pollution, with varying levels of toxic effects adding to the overall impact. Oils, unlike most forms of water pollution, can also have damaging effects on terrestrial organisms, such as plants, birds and mammals living at the water's edge, since heavy oil contamination usually lead,, to deposition of oil on the banks of likes or rivers.

Generally, the use of oil dispersants, which are commonly used in dealing with marine spills, is avoided in freshwater because of their high toxicity. Also, the physical conditions of freshwater habitats are often amenable to containment and recovery of oil by booms, skimmers and/or tile use of absorbent materials.

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