Minecraft wiki entry for River, associated text: Rivers are thin, lengthy biomes that frequently generate across the world. They typically serve as a division between two different biomes, but can also split a single biome in two. They are filled with water up to sea level, though rivers may run dry in certain areas due to higher than usual elevation. Exposed grass in rivers takes on a bluish-green hue and oak trees can rarely generate along with spare grass. Riverbeds consist of dirt, sand, gravel, and clay, and are one of the few places where clay generates, alongside lakes, swamp marshes, ocean hilltops, and lush caves. Seagrass generates atop the riverbed, and sugar cane occasionally generates on the riverbank. Neither passive nor hostile‌[BE only] mobs (except drowned) spawn within river biomes themselves, but frequently wander into them from the surrounding biomes if they can spawn there. Drowned can spawn underwater, as can salmon and squid. Rivers can vary a lot in width and depth, in generally flat areas rivers are usually around 10 blocks deep but in mountainous regions rivers can reach over 30 blocks in depth which is comparable to deep oceans. Rivers in mountains ranges can for fjords or be replaced by another biome forming a saddle valley. Sometimes rivers can also generate as lakes with small islands.