The start of a river is called: the SOURCE.
Glaciers in high mountain areas melt. This water starts flowing and becomes a river.
Weathering mostly occurs at this part of the river, called the UPPER course. Large rocks are slowly broken down into smaller rocks.
One type of weathering is when roots of PLANTS AND TREES grow through ROCKS
The second type of EROSION takes place when WATER gets into cracks and crevices.