The process by which new land formations are legally assimilated to old by a change in the flow of a water channel. In contrast to avulsion, this process involves a very slow, near imperceptible, natural action of water and other elements. It would include, for example, the natural diversion of a boundary river leaving an island, sandbank, or dry land where it previously flowed, the formation of islands at a river mouth, and additions to a delta by the deposit of sand and soil upon the shoreline. Accretion will allow the beneficiary state to legitimately claim title to the new land so created. See also thalweg, rule of the.
For cases in which acquisition of territory by accretion has arisen, see The Anna (1805) 5 C Rob 373 (Lord Stowell) and Secretary of State for India v Chellikani Rama Roa (1916) 32 TLR 652 (PC). There is a large volume of US decisions concerning accretion; the classic illustration of a finding of both accretion and avulsion can be found in the Chamizal Arbitration 5 AJIL 785 (1911) concerning the path of the Rio Grande.