Such changes include increased run-off from impervious surfaces and piped tributaries, stock access, or direct interference such as straightening or channelling the stream. If it has been channelled or stabilised with concrete bank s, the stream will obviously be stable with little erosion , but should not be ranked highly as it has no vegetation cover or a greatly reduced one. 5. Riffles, pools and bends Rocks and debris in the stream may create shallow areas over which the water rushes quickly to form a rapid. This is called a riffle in the stream system. Upstream of a riffle the water is often quiet and may form a pool.
Pools are important in providing deeper areas for fish.
Streams that have a number of pools and riffles are able to support more life and greater variety of species than those that do not vary in character at all. Larger, slow-flowing rivers may not have riffles, but bends in the river can provide different habitats because the cutting action of the water at bends provides deeper areas and areas of different water speed. The proportion of pools, riffles and bends varies naturally between major, minor and tributary streams, so the rating categories provide descriptions for each of the different stream types. Record your rankings on the Stream Habitat Record Sheet. A number score has also been given to each ranking, which will allow you add up all the factors you survey and obtain a total score for the particular site.