Flowing Water as in Rivers and Streams:

The water from rivers and streams etc., contains silt, sand and suspended impurities, beside containing treated or untreated sewage from upstream end. Therefore, water from these sources is considered as less satisfactory in quality than that from lakes or impounding reservoirs. The quality of water is also subject to the widest variation depending on the factors such as property of the catchment area, its climate, geological and topographical features, the extent to which it is subject to contamination from surrounding population, the seasonal variations and weather conditions. Substantial variation in the quality of the water may also occur from point to point along the course of same river. Normally the quality of water deteriorates as stream/river proceeds along its course. The water quality may also vary with maximum and minimum flow conditions.

The perennial and large rivers and streams constitute the main source of water supply scheme. However, the quantity of water available from non- perennial rivers and streams may vary throughout the year depending on weather conditions. It normally falls down during summers when water demand is maximum. Therefore, these are generally not considered as good sources of water supply schemes but may be adopted by providing storage reservoirs and barrages etc. They augment it with other sources of water supply.

The question of the flow through streams may be divided into three parts:

(a) The minimum flow through the stream

(b) The maximum or flood flow

(c) Variation in the flow through successive months and year. The first information is necessary in case a stream is under consideration from which little storage is obtainable or in answer to the question whether it is practicable to draw water directly from the stream without storage. It is also important in problems of water power where power is to be furnished continuously throughout the year.

The second point is of importance in the design and execution of all river work especially in determining size of waste weirs.

The third determines the supplying capacity of the water shed and the size of the impounding reservoirs.