Riverine system of India

Characteristics of Riverine system

Rivers are linear systems, which show a gradient of characters along their length. The longitudinal profile of a river is concave with a steep upper portion near the source, giving way to reaches of progressively less gradient as the mouth is approached.

• The steep and torrential upper course is called “rhithron” and
• The flat, slow-flowing lower course is called “potamon”

Rhithron zone

Rhithron zone characteristics

They tend to show an alternation between

• Steep, narrow and shallow riffles or rapids,
• Flatter, wider and deeper reaches, termed pools.

Turbulent flow and relatively low temperatures characterize this zone. Normally, the water is highly oxygenated, but at low water, the pool and riffle system may break up into a series of pools, whose waters may become completely depleted of oxygen.

Adaptations of fishes of rhithron zones

Fish species in rhithron zones are entirely rheophilic and are categorized into two groups.

• The species which lives on or among the rocks and vegetation of the bottom, are distributed predominantly in the riffles. Typically, these are small size and are adapted to grip or cling to the substrate. For such adaptation including the mouth suckers for instance Chiloglanis, ventral friction pads as in Astroblephus or pectoral fin spines adapted as hooks as in Glyptothorax.
• The species such Barbus or salmo are adapted to swim sufficiently fast as resist to the current and even against it. This they cannot do on a sustained basis, however, and frequently take advantage of cover provided by the slack water of the pools and by snags, overhangs, and other features, which disrupt the current. Because of the severity of the habitat, diversity of resident species tends to be low.

Potamon zone

Potamon zone Characteristics

• Potamon reaches with wide, flat, meandering channels, mud bottoms and much rooted and floating vegetation. Zonation within the potamon is both longitudinal and lateral. Longitudinally, there is a repetition of differing habitats associated with the meanders of the channel. Laterally, there is the distinction between the main channel and its floodplain. The floodplain is usually an area of relatively flat land flanking the main channel. In exceptional cases, larger floodplain areas arise by geographic accident. The plain is usually higher near the river, where raised levees limit the main channel, and slopes downward toward the foot of the terrace confining the plain. Many bodies of water are found on the plain ranging from small temporary pools to large permanent lagoons and swamps.
• The potamon is environmentally more complex than the rhithron. There is usually a well-defined series of river channels flanked by a floodplain. Both the running and still waters may be present. The plain itself contains many types of the water body, some of which retain water throughout the inter-flood period. Because of deposition of silt, such features show a succession from the open lagoon, through vegetation-lined pools and densely vegetated swamps to dry land. In the water bodies of the floodplain, dissolved oxygen concentrations fall in the dry season, particularly in the smaller pools, which may become completely depleted of oxygen.

Adaptations of fishes of potamon zones

There are two main adaptations, which enable fish to survive the conditions during low waters.

• The Species, which is specially adapted to resisting the low dissolved oxygen concentration. The adaptations may be in the form of auxiliary respiratory organs for using atmospheric oxygen i.e. Clarias or Notopterus, or may be physiological as with Carassius or even behavioural as with many cyprinodonts. The same species have a capacity to tolerate high temperatures. They have
• complex breeding habits with multiple spawning,
• a great degree of parental care, and
• migrate laterally between the dry season habitat in the river channel and the flood season habitat in the inundated area.
• Species, which uses the rich habitat provided by the flood plain during the flood but escapes the severe dry season conditions by lateral movement off the plain and longitudinal migration within the main river channel to an alternative habitat. This is usually located in the deeper regions of the main river channel, but may also be in the sea or some other large water body adjacent to the river system. A certain proportion of these species moves upriver, even as far as the rhithron zone. Such fishes show few adaptations other than a capacity for fast and sustained swimming. Their breeding strategy is simple, relying on a single release of a vast number of eggs, either on the flood plain or in the headwater streams. To accomplish this, they may undertake migrations for very long distances up-and down-river.

Riverine system classification

In India, rivers play a major role in the livelihood of the peoples. The riverine system is categorized into four groups namely

• Major Rivers
• Medium Rivers
• Minor Rivers and
• Desert Rivers.

Most of the rivers pour their water in the Bay of Bengal and some of them though pour in the western parts of India Arabian Sea.

Major rivers

Major rivers are those which has a catchment area of 20000 km². There are 14 such major rivers in India. Further, they are classified into Himalayan Rivers and Peninsular rivers based on the origin of the river.

Medium rivers

A river with a catchment area of 2000 - 20000 km² is categorized as Medium River. There are 44 such rivers in India, with a total drainage area of 0.24 million square kilometers. Of these rivers, nine rivers are interstate rivers as they flow through more than one state. 17 rivers flow towards the Arabian Sea, 23 flow towards the Bay of Bengal and Four rivers in North Eastern states.

Medium river basins

Minor rivers

Minor rivers are those which has the catchment area of less than 2,000 km². There are numerous and mostly small streams flowing from Western and Eastern Ghats to the sea. The total drainage area is about 0.2km².

Desert Rivers

These rivers flow for some distance and disappear in the deserts of Rajasthan. These rivers include Luni, Machai, Rupen, Saraswati, Baner and Ghaggar. The  Amblypharyngodon mola, Bari bendelansis, Botia geto, Catla catla, Cirrhina mrigala, C. reba, Labeo spp, Danio devario, D. rerio, Tor tor, Nemacheilus botia, Puntius spp, Rasbora daniconius, Mastacembelus armatus, Channa spp., Trichogaster fasciatus, Clarias batrachus, Mystus spp, Heteropneustes fossilis, Wallago attu, Notopterus spp.,are the major fishes.