13.5 TRENDS IN RIVER DEVELOPMENT

Most of the major rivers of the tropics are following a similar developmental path. These changes involve:

• Progressive deforestation and farming of hillslopes and other marginal lands in the basin, leading to increased flashiness of hydrological regimes and increased silt loads.

• Progressive damming in the interests of power generation, and increasingly for water transfers and irrigated agriculture. This process is transforming many tropical rivers, including changing tributaries into cascades of reservoirs.

• Channelization of the main river in the interests of navigation, flood control and reclamation of the floodplains for agriculture, by channel straightening and deepening, and levee construction.

• Poldering of floodplains in the interest of improved control of the hydrological cycle for rice culture.

• Increasing eutrophication from urban and agricultural discharges, and pollution from industry.

Not all rivers undergo all these processes at the same rate. However, it is worth noting that many of the great temperate rivers were modified in the ways described above over the past two or three centuries, but there are now trends to reverse this process and rehabilitate what have become very damaged systems. In fact, the trend in some North American rivers is for removal of former dams on major river systems, and the gradual return of the rivers to a more 'natural' state.