tributary

A tributary is a stream or river that flows into a larger river or lake. It does not flow directly into the ocean, unless it is part of a larger river system. A tributary contributes water, sediment, and sometimes nutrients to the larger water body, influencing hydrology, ecology, and flood dynamics. The main river plus its tributaries form a drainage basin, or watershed, a network that drains rainfall and snowmelt. The point where a tributary meets the main stem is called a confluence; tributaries join the river along its course and increase discharge.

Tributaries can be perennial, flowing year-round, or intermittent, seasonal. They vary in size from tiny streams to large rivers, and are often named differently in local geography. Tributaries are classified by their relationship to the main stem: major tributaries are typically large inflows that contribute substantial discharge; minor tributaries add smaller inflows.

In river deltas, distributaries emerge from the main stem and carry water away toward the sea, opposite of a tributary. In hydrology, tributaries influence flood wave propagation, sediment load, and nutrient transport downstream. Erosion and deposition at confluences create varied channel morphology and river terraces over geological time.

Examples include the Mississippi River system, fed by major tributaries such as the Missouri and Ohio rivers, and the Amazon, with numerous tributaries such as the Rio Negro and Tapajós. The term is widely used in hydrology, geography, and environmental planning.