Short answers, pulled from the story.
A river is a natural stream of fresh water that flows on land or through caves toward a lower body of water such as an ocean, lake, or another river. All the water in rivers ultimately comes from precipitation, arriving as rainfall, runoff, melting glaciers and snow, or seepage from underground aquifers.
Rivers shape the land by carrying alluvium and eroding rock into canyons, valleys, and waterfalls. They bend into meanders because obstacles deflect the current, alluvium builds against the obstruction, and the flow erodes the opposite bank into a concave curve before reflecting back the other way.
The first civilizations were born on floodplains between 5,500 and 3,500 years ago because rivers provided fresh water, fertile soil, and transportation. The Sumerians arose in the Tigris-Euphrates system, the Ancient Egyptians along the Nile, and the Indus Valley Civilization on the Indus River.
Humans engineer rivers for flood control, navigation, recreation, and ecosystem management using dams, dikes, levees, channelization, and dredging. Dams restrict flow to raise water for boats, generate hydroelectricity, and create reservoirs, with at least 75,000 dams taller than 6 feet in the United States.
Rivers are threatened by water pollution, climate change, and human activity such as dam construction, which blocks fish migration and lowers alluvium flow. Freshwater fish make up 40 percent of the world's fish species, and 20 percent of these species are known to have gone extinct in recent years.
There is evidence that rivers flowed on Mars for at least 100,000 years, with channels and deltas visible in Hellas Planitia, where sedimentary rock formed 3.7 billion years ago. On Saturn's moon Titan, channels called flumina may carry liquid methane and ethane.