The document describes the formation of deltas. As a river reaches the sea, it deposits sediments, building up the river mouth and causing the river to split into distributaries. Over time, accumulated sediments rise above the water level, forming a delta where the river meets the sea. It also discusses how meanders form through a river's winding course as it seeks to avoid obstacles at a low gradient, eroding the outer concave bank and depositing sediments to form slip-off slopes on the inner convex bank. Rivers provide benefits like water and food but can also negatively impact people through flooding and spreading disease.