Rivers grow bigger when tributaries (smaller streams) join the main river.
Merging rivers have different colours if they are carrying different amounts and kinds of sediment .
When it rains, some raindrops soak into the ground. Other raindrops coalesce (stick together) and flow over the surface of the ground in tiny rills and runnels before finding their way into streams and rivers. This is called overland flow. You can see overland flow happening in places where there aren't many plants covering the ground.