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Rivers have been important throughout the history of exploration because they have provided one of the easiest means of moving through undeveloped country.
The amount of water in a river is a balance between what flows
in and what flows out. The water flowing in comes mainly through the soil and the rock below the river bed. Very little of it falls directly onto the river as rain. In mountain areas some water may also come from melting snow and ice. During a heavy storm rain may fall faster than it can seep into the soil, and then it will flow over the surface and into rivers.
Rivers may lose water as they flow over permeable rocks and through evaporation. In developed areas large losses may also be due to the removal of water for human use.
Rivers flow in two patterns—a single winding channel, called
a meandering channel (see: Meander), and a network of intertwined channels, called a braided channel.
(For other natural river features see: Alluvial fan; Basin; Branch; Cataract; Cliff; Confluence; Delta; Drainage basin; Estuary; Ford; Fork; Headwater; Oxbow, oxbow lake; Rapids; River channel; Tributary; Waterfall.)
River channel
The trench in which a river flows for most of the year.
River channels change shape mainly during floods, when the channels are full of fast-flowing water, and the available energy is high. That happens on average about once every two or three years. For the rest of the time little change occurs. Two types of channel are the most common:
deep, winding or meandering channels that occur in places
where the majority of the material carried is fine sediment such as silt and clay; and shallow, wide, braided channels that occur where the material is mainly sand, gravel, or cobbles.
S
Sand
A type of sediment carried by flowing water. It is between 0.06mm and 2mm across.
Sand
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Runoff
Water that reaches a river as a result of rainfall or snowmelt in the river basin. It is part of the water cycle.
Runoff is mostly unseen, occurring as seepage through soil and rocks. Rain only flows directly to rivers after torrential rain or very long periods of rain. That is when floods occur.
Surface runoff is now much more common than it was in the past, mainly because people change the natural process that allows runoff to soak into the ground by
covering the ground with pavement and buildings. In such developed areas there is no chance for water to seep into the soil, and all of the rainfall becomes runoff. Moreover, the roofs and streets of cities are smooth and so do not hold back water.
To collect the water from roofs and streets, a complicated network of underground waterpipes is
used. They are often called storm drains. Many such drains carry water directly to rivers, and as a result rivers fill with water far more easily than in the past.
Under-road storm drain
Inspection cover
House storm drain
 Runoff—How house rainwater drains help speed runoff to rivers and increase the chance of flooding.