Balanced ecosystems comprising soil, water and plant environments are essential for the survival and welfare of mankind. However, ecosystems have been disturbed in the past due to over exploitation in many parts of the world, including some parts of India. The resulting imbalance in the ecosystem is revealed through various undesirable effects, such as degradation of soil surfaces, frequent occurrence of intense floods etc. For example, the large scale deforestation which occurred in the Shiwalik ranges of the Indian Himalayas during the 1960s caused the soil on the land surfaces to be directly exposed to the rains. This unprotected soil was readily removed from the land surface in the fragile Shiwaliks by the combined action of rain and resulting flow. Vast tracts of land over the country have been irreversibly converted into infertile surfaces due to accelerated soil erosion caused by the above and other factors. These degraded land surfaces have also become a source of pollution of the natural water. Deposition of soil eroded from upland areas in the downstream reaches of rivers has caused aggradation. This has resulted in an increase in the flood plain area of the rivers, in reduction of the clearance below bridges and culverts and in sedimentation of reservoirs. In the present paper a review and analysis of the problems caused by soil erosion and associated sediment deposition in India is presented. These problems are first discussed in detail. Possible remedial measures are then discussed in brief at the end of the paper. 

PROBLEMS CAUSED BY SOIL EROSION 

In India a total of 1 750 000 km2 out of the total land area of 3 280 000 km2 is prone to soil erosion. Thus about 53% of the total land area of India is prone to erosion (Narayana & Ram Babu, 1983). Areas affected by soil erosion in India can be broadly grouped into two categories, representing, firstly, the Himalayan and Lower Himalayan region and, secondly, other regions. 

The Himalayan and Lower Himalayan region Most parts of the Himalayas, particularly the Shiwaliks which represent the foothills of the Himalayas in the northern and eastern Indian states, are comprised of sandstone, grits and conglomerates with the characteristics of fluvial deposits and with deep soils. These formations are geologically weak, unstable and hence highly prone to erosion. Accelerated erosion has occurred in this region due to intensive deforestation, large scale road construction, mining and cultivation on steep slopes. Approximately 30 000 km2 have been severely eroded in the northeastern Himalayas due to shifting cultivation (Narayana & Ram Babu, 1983). 

Deforestation and associated soil erosion has caused desertification of land in the Shiwalik hills in the Hoshiyarpur district of the Punjab state. The extent of degraded land in this area was 194 km2 in 1852, 2000 km2 in 1939, while it increased to 20 000 km2 in 1981 (Patnaik, 1981). Similarly, large tracts of cultivable land have been abandoned because of the erosion of topsoil in the Kotabagh area of the Nainital district in the state of Uttar Pradesh (Valdiya, 1985). In addition about 45 % of the perennial hill springs in these areas go dry during the non-monsoon season because of the reduction in groundwater storage resulting from the erosion of the pervious soil horizons (Valdiya, 1985). Landslides are the other dominant cause of soil erosion and related problems in the Himalayas. Figure 1 provides a map of the landslide prone areas of India (Garde & Kothyari, 1989). This map indicates that in India landslides mainly occur along the Shiwaliks in the Himalayas. The landslide prone areas also coincide with the locations of high magnitude earthquakes, geological faults and values of the rainfall event ratio (E) which indicate the occurrence of intensive rainfall for long durations. In addition to soil erosion, landslides also cause various other problems, such as the creation of embankments across streams. The breaching of such barriers across streams created by landslides has had disastrous effects, as illustrated by the following example from the Alaknanda valley (Ravindra & Negi, 1982). An intense thunderstorm over the Alaknanda valley in the Himalayas during 1970 led to severe erosion/landslides and breaching of the barriers which had developed across the Alaknanda River and its tributaries. The sediment load thus increased enormously and the concentration of sediment in the Ganga River at Hardwar (The Alaknanda and the Bhagirathi join at Deoprayag about 80 km upstream of Hardwar to form the Ganga, see Fig. 2) was about 34 000 mg l\"1. Thus the Upper Ganga canal which diverts water from the Ganga at Hardwar had to carry a very heavy sediment load. The consequences of the heavy sediment load entering the canal are discussed below.