曲流（又称河曲或河套）是指河流的弯曲。曲流多见于下游。河道的外弯水流较快，河岸受侵蚀；内弯则水流较慢，沉积物累积形成新的河岸。河道因此越来越弯，河谷亦越来越宽。当曲流过弯而脱离主河道，该段曲流就会变成牛轭湖。

A meander, in general, is a bend in a sinuous watercourse or river. A meander forms when moving water in a stream erodes the outer banks and widens its valley, and the inner part of the river has less energy and deposits silt. A stream of any volume may assume a meandering course, alternately eroding sediments from the outside of a bend and depositing them on the inside. The result is a snaking pattern as the stream meanders back and forth across its down-valley axis. When a meander gets cut off from the main stream, an oxbow lake forms. Over time meanders migrate downstream, sometimes in such a short time as to create civil engineering problems for local municipalities attempting to maintain stable roads and bridges.