Without doubt, man has been the most prodigious of all agents of change to affect streams in the last few centuries. With the invention of agriculture some 10,000 years ago, human colonization rapidly moved from the foothills to the more fertile river valleys. Ever since that move, man has been trying to control rivers, especially by attempting to control floods and sedimentation. Being in a state of dynamic equilibrium rivers, in turn, react to human interference. 

One common practice has been to dam rivers to store larger water supplies, creating new lakes. These reservoirs slowly fill with sediments, eventually defeating the very purpose for which they were created. Downstream, the valley begins to erode because the water no longer has the sediment load to which its course was adjusted. The silts and clays that used to replenish the fertility of the floodplain are absent. In turn, continued fertility of the bottomlands necessitates chemical fertilizer application. Many of these chemicals now seep into the rivers and cause pollution. 

Channelization, and the building of control structures designed to decrease flood hazards, do indeed decrease the frequency of small floods. Unfortunately, when breached, these structures also impound water in the flood plain, preventing it from flowing back into the river after the flood crest has passed, thereby increasing the severity and duration of large floods. This also decreases the floodplain's ability to absorb chemicals and deposit nutrients in the backswamps. 

Modern ploughing practices have increased runoff and tripled the sediment load that rivers must handle. Deforestation and urbanization have also increased runoff and soil loss and aggravated the magnitude and frequency of floods. 

The list of mistakes that man has made as a result of misunderstanding and neglect of river dynamics is almost endless. Only of late have we begun to understand the role which running water plays in the ecosystems of the world and have we come to realize that whatever is done to a river in one place ultimately affects all living things along its course. 

Creates divides, sealevel rises and lowerings; adjust base level. Relationship between continental size and height.