River

Rivers are common technical biomes. They serve as borders between various other biomes, and usually lead to oceans, though they can occasionally form circles.

Description
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. Riverbeds consist of dirt, sand, gravel, and clay, and are one of the few places where clay generates, alongside lakes, swamp marshes and ocean hilltops. Seagrass generates atop the riverbed, and sugar cane occasionally generates on the riverbank. Neither passive nor hostile 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.

When generated between two high-elevation biomes, such as mountains and hills, river biomes may pose a fall damage hazard, but are otherwise mostly harmless.

If this biome is used to generate a buffet world, it will look like a shallow ocean, of no more than 10 blocks deep, with mostly sand, dirt, clay and gravel floor and infrequent grassy islands that rarely contain oak trees and, more rarely, pumpkins.

Variants
There are two different river biomes.

River
The standard river variant. Salmon and squid may spawn here.

Frozen River
Frozen Rivers replace regular rivers, and only generate within snowy tundra biomes. As the name implies, the top layer of water is entirely frozen, though below the ice is the usual sand, gravel, and clay, though seagrass does not generate on the riverbed. While sugar cane can generate alongside a frozen riverbank, it quickly uproots itself after generation due to the water being frozen. Along with salmon and drowned, strays, rabbits, and polar bears can spawn in frozen rivers. No other hostile mobs can spawn, even underground.

Note that frozen rivers can only cut through snowy tundra, not its variants; if a river generates separating the snowy tundra from one of its variants or any other biome, it will only be partially frozen.

Trivia

 * Lakes can technically still generate in the waters of rivers and frozen rivers, when either are used as buffet worlds. This can be more clearly seen in frozen river worlds, as holes in the ice.
 * Regular rivers can still generate cutting through frozen rivers.
 * Both types of rivers override ravines at or close to the surface level, causing the ravine to be abruptly cut off by a wall of stone. However, if the ravine is long enough, it may continue on the other side of the river.
 * Oddly ravines can generate underground in river biomes without being cut off.