River

A river is a common biome. Rivers can serve as borders between various other biomes, and usually lead to oceans, although they can also form loops.

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 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 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. 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.

When generated between two high-elevation areas, such as mountains and hills, river biomes may pose a fall damage hazard, and deep rivers can also spawn drowneds frequently, but are otherwise mostly harmless.

Variants
There are two different river biomes.

River
The standard river variant. Salmon and squid may spawn here. This is the third most common biome in the game, only behind plains and forest.

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Frozen River
Frozen Rivers replace regular rivers in regions with snowy biomes. As the name implies, the top layer of water is entirely frozen, though below the ice is the usual sand, gravel, dirt, 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. Oak trees can still generate here if this biome generates on land, despite being a snowy biome.

$$, along with salmon and drowned, strays, rabbits, and polar bears can spawn in frozen rivers. No other hostile mobs can spawn, even underground.

$$, frozen rivers use the same mob spawning chances as rivers for water and ambient categories. As for the others:

$$, frozen rivers use the same mob spawning chances as rivers for water categories. As for the others:

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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.