1BASIC CONCEPTSAll land is part of a watershed or river basin.Figure 1 shows the landscape sculptured by erosion in Greece and the very dry land in Egypt. Indeed，rivers are such an integral part of the land that in many places it would be as appropriate to talk of riverscapes as it would be of landscapes. Rivers are much more than merely water flowing to the sea. Rivers carry downhill not just water，but just as importantly sediments，dissolved minerals，the nutrientrich detritus of plants and animals. Their evershifting beds and banks and the groundwater below are all integral parts of rivers. Even the meadows，forests，marshes and backwaters of floodplain can be seen as part of the rivers—and the rivers as part of them.

Figure 1(a) The landscape sculptured by erosion (Greece)； (b) river flow is the main drive for development of geomorphology (Norway)

The main functions of rivers are draining floods，supplying drinking water，maintaining ecology，irrigating farmland，transporting sediment，supplying power，providing habitat for fish，assimilating wastewater，and providing navigation. Humans exploit the resources of rivers by constructing dams and waterdiverting channels，developing navigation channels，and harvesting fish，which result in changes in the river hydrology，runoff，sediment transport，riparian and stream habitats，and water quality. Watersheds start at mountain peaks and hilltops. Snowmelt and rainfall wash over and through the high ground into rivulets which drain into fastflowing mountain streams. As the streams descend，tributaries and groundwater add to their volume and they become rivers. As they leave the mountains，rivers flow and start to meander and braid，developing channels within the valleys with alluvial floors laid down by millennia of sedimentladen floods. Eventually rivers will flow into a lake or ocean. Where the river carries a heavy sediment load and the land is flat，the alluvial sediments may form a delta. Estuaries，the places where the fresh water of rivers mix with the oceans salt，are among the most biologically productive parts of rivers and of seas. Most of the worlds fish catch comes from species that are dependent for at least part of their lifecycle on a nutrientrich estuarine habitat.Figure 2 shows the components of a river system，materials transported，and the aspects affected by the rivers and transported materials. Rivers can be recognized as mountain rivers，alluvial rivers，and estuaries. A mountain river is the most upstream part of the river，including the river source and the upstream tributaries of the river，where the river system flows through mountainous areas and the flow is confined by mountains. Usually the channel bed of a mountain river is composed of gravel. Mountain rivers receive most of the sediment，nutrientrich detritus of plants and animals，dissolved materials，and usually more than half of the water.

Figure 2River system，transported materials，and aspects affected by rivers and the transported materials

For a large river the upstream reaches compose the inputpart of the river and are directly affected by the watershed or drainage area. Erosion control and vegetation development are the most challenging tasks for researchers and watershed managers. Erosion induced landslides and debris flows are disastrous in the upstream reaches. Mountain rivers are quite often incised rivers and degradation of the channel bed causes many problems. Therefore，erosion control and vegetation development over the watershed，landslides and debris flows，and control of channel bed incision are major topics of mountain river studies.An alluvial river is defined as a river with its boundary composed of the sediment previously deposited in the valley，or a river with erodible boundaries flowing in selfformed channels. Over time the stream builds its channel with sediment it carries and continuously reshapes its cross section to obtain depths of flow and channel slopes that generate the sedimenttransport capacity needed to maintain the stream channel. Alluvial rivers are mostly perennial streams and the channel bed is composed mainly of sand and silt. A large river usually originates from mountains and flows over floodplains before it pours into the ocean，therefore，it is a mountain river in its upper reaches and an alluvial river in its lower reaches. Many alluvial rivers are large rivers or flatland sections of large rivers，such as the lower reaches of the Yellow River and the middle and lower reaches of the Yangtze River.These alluvial rivers are confined within the valley defined by human constructed or artificially reinforced levees. The river morphology and river patterns depend mainly on the sediment transportation and deposition.Rivers are the main source of water resources for agriculture，urban use，and industry. River flood are major natural disasters accounting for 1/3 of the total loss due to natural hazards. The quality of river water is important for human health. Flood and sediment transportation are natural processes in these rivers and water diversion，channelization，and navigation are human disturbance，to the rivers. Thus，sediment transportation，water resources development，and flood defense are the most important issues in the alluvial river management. The estuary is the connection part of a river with the sea or the ocean into which it flows，including the river mouth，a river section affected by the tide，and the water body area affected by the river flow. Sediment is deposited for land creation and very often a delta develops in the area. In recent years，the need for sustainable development of coastal cities and marine resources has given rise to challenging environmental problems. Examples include the environmental impact assessment of dredging and sludge/spoil dumping and the transport and transformation of nutrients and heavy metals at the sedimentwater interface. Urban development including largescale land reclamation and population growth induced increase in sewage discharge puts the estuary ecosystem under stress. Red Tide is a phenomenon in which the seawater is discolored by high algal biomass. Some algal species produce potent toxins，which accumulate in shellfish that feed on those algae，resulting in poisoning in human consumers. There has been a significant expansion of red tide episodes and impacts throughout the world over the last several decades. Very unusual red tides have occurred in the Bohai Sea，East China sea，and South China sea in the past decades. Delta and coastal processes，eutrophication，and algal blooms are the major challenges for the management of estuaries.A variety of riveruses was the driving force of societal development in the past and now is even more important in economic and cultural development. Rivers，and the rich variety of plants and animals which they sustain，provided huntergatherer societies with water for drinking and washing，and with food，drugs and medicines，dyes，fibers，and wood. Farmers reap similar benefits as well as，where needed，irrigation for their crops. For pastoral societies，who graze their herds over wide areas of often parched plains and mountains，perennial vegetation along the banks of rivers provides lifesustaining food and fodder during dry seasons and droughts. Towns and cities use and misuse rivers to carry away their wastes. Rivers also served as roadways for commerce，exploration，and conquest. The role of rivers as the sustainers of life and fertility is reflected in the myths and beliefs of a multitude of cultures.