Limnology

Unit 1- Limnology- An introduction

1.3.3. Streams and Rivers
A. STREAMS
Introduction
Streams are zones where a rapid flow of shallow water produces a shearing stress on the stream bed, resulting in a rocky or gravel substratum covered by fully oxygenated water. Streams may vary in size from tiny rivulet to rivers. As time goes the stream may develop into river or increase its size, whereas the size of reservoirs decreases as time passes. They are more numerous in regions of abundant rain fall. They are temporary or permanent. Streams are closely linked to their watersheds. The productivity of streams is often dependent on terrestrial bases, grasses and other debris. The allocthonous materials contribute most of the food and energy to the organisms in the stream. Benthic invertebrates like insect larvae constitute the invertebrate fauna. True plankton are almost absent in streams, and are common only in deep slow moving stretches of rivers. All biota in streams are influenced by the unidirectional current.


Physical conditions
The annual change in stream temperature is 10 to 20°C. Although large rivers do not change in temperature very much on a daily basis, a small unshaded stream may heat up to 10°C in a few hours on a hot summer’s day and cool by the same amount at night. The temperature of most streams is lowest in the upland and becomes gradually warmer in the lower reaches.
The velocity of stream water varies with the landforms. In plains, streams are slow and sluggish throughout their length. In mountain stretches the speed of water may be rapid.
Stream water has uniform temperature and the difference between the surface and bottom is virtually negligible. The stream follows air temperatures more closely than lake waters and the factors responsible are depth of water, current velocity, bottom material, temperature of entering water, exposure to direct sunlight and degree of shading etc.
Extreme of turbidity occur in running water series and streams with rock beds the turbidity is minimal.
Stream systems increase their length, width and depth with increasing age. This is in distinct contrast to the reduction processes characteristic of all standing water units.
At any position along the course of a running water system, materials eroded at that point and all materials suspended or dissolved at the level are transported downstream with no opportunity to return. Interchanges of materials are more and have less depth than lakes.
Chemical conditions
The dissolved oxygen supply in uncontaminated stream is high at all levels often near saturation. The polluted streams show low dissolved oxygen due to accumulation of organic wastes. Stream which support more plants show diurnal variation of dissolved oxygen. The level of dissolved oxygen is controlled by the slope of channel and mode of flow.
Current in streams tends to keep the pH in uniform over considerable distances. It keeps any acidity due to accumulating free CO₂ reduced. Streams waters do not develop the more intense acidities.
The dissolved solids of streams are affected by their irregular discharges. Most streams and rivers have maximum discharge during winter rains, particulate matters, nutrients like phosphate, iron and nitrate are transported to different parts by the flow of the streams. Streams fed by springs have more constant nutrients.

B. RIVER
River is said to be a natural stream of water usually fresh water flowing towards an ocean. In some cases river flows into the ground or dries up completely before reaching another body of water. Usually larger streams are called rivers while smaller streams are called creeks, brooks, rivulets, rills, and many other terms.
A river is a component of the hydrological cycle. The water within a river is generally collected from precipitation, through surface run off, ground water recharge and release of stored water in natural reservoirs such as glacier.
Topography
The water in a river is usually confined to a channel, made up of stream bed between banks. In larger rivers there is also a wider floodplain shaped by flood waters over-topping the channel. Flood plains may be very wide in relation to the length of river channel. This distinction between river channel and floodplain can be indistinct especially in urban areas where the floodplain of a river channel can become greatly developed by housing and industry.
Ecology
The flora and fauna of rivers use the aquatic habitats available, from torrential waterfalls through to lowland mires although many organisms are restricted to the freshwaters in rivers eg salmon and Hilsa.
Flooding
Flooding is a natural part of river cycle. The majority of the erosion of the river channels and the erosion and deposition on the associated flood plain occur during flood stage. Human activity, however has upset the natural way flooding occurs by walling of rivers set straight their courses and by draining of natural wetland.