Rivers have been used by man since the dawn of civilization as a source of water, for food, for transport, as a defensive barrier, as a source of power to drive machinery, and as a means of disposing of waste.

For thousands of years rivers have been used for navigation (The earliest evidence of navigation is found in the Indus Valley Civilization, which existed in north-western India around 3300 BC). Riverine navigation provides the cheapest means of transport and is still used extensively on major rivers of the world like the Ganges, the Nile, the Mississippi, and the Indus.

In some highly-forested countries like Scandinavia and Canada, lumberjacks use the river to float felled trees downstream to lumber camps for further processing, saving much effort and cost by transporting the huge heavy logs by natural means.

Rivers have been a source of food since pre-history. Apart from being a rich source of fish, rivers indirectly aid cultivation by supplying water for the crops. Rivers sustain their own food chain. They are a major source of fresh water, hence, it is no surprise to find most of the major cities of the world situated on the banks of rivers. Rivers also provide an easy (if unsanitary) means of disposing of waste.

http://en.wikipedia.org/wiki/River#Use_of_rivers