Poor as a scientific boundary, a river is not satisfactory even as a line of demarcation, because of its tendency to shift its bed in every level stretch of its course.  A political boundary that follows a river, therefore, is often doomed to frequent surveys.  The plantations on the meanders of the lower Mississippi are connected now with one, now with the other of the contiguous states, as the great stream straightens its course after the almost annual overflow.[702] The Rio Grande has proved a troublesome and expensive boundary between the United States and Mexico.  Almost every rise sees it cutting a new channel for itself, now through Texas, now through Mexican territory, occasioning endless controversies as to the ownership of the detached land, and demanding fresh surveys.  Recent changes in the lower course of the Helmund between Nasralabad and the Sistan Swamp, which was adopted in 1872 as the boundary between Afghanistan and Persia, have necessitated a new demarcation of the frontier; and on this task a commission is at present engaged.[703] In a like manner Strabo tells us that the River Achelous, forming the boundary between ancient Acarnania and Aetolia in western Hellas, by overflowing its delta region, constantly obliterated the boundaries agreed upon by the two neighbors, and thereby gave rise to disputes that were only settled by force of arms.[704]

[Sidenote:  Fluvial settlements and peoples.]

Rivers tend always to be centers of population, not outskirts or perimeters.  They offer advantages that have always attracted settlement—­fertile alluvial soil, a nearby water supply, command of a natural highway for intercourse with neighbors and access to markets.  Among civilized peoples fluvial settlements have been the nuclei of broad states, passing rapidly through an embryonic development to a maturity in which the old center can still be distinguished by a greater density of population.  Only among savages or among civilized people who have temporarily reverted to primitive conditions in virgin colonial lands, do we find genuine riverine folk, whose existence is closely restricted to their bordering streams.  The river tribes of the Congo occupy the banks or the larger islands, while the land only three or four miles back from the stream is held by different tribes with whom the riverine people trade their fish.  The latter are expert fishermen and navigators, and good agriculturists, raising a variety of fruits and vegetables.  On the river banks at regular intervals are market greens, neutral ground, whither people come from up and down stream and from the interior to trade.  Their long riparian villages consist of a single street, thirty feet wide and often two miles long, on which face perhaps three hundred long houses,[705] Fisher and canoe people line the Welle, the great northern tributary of the Congo.[706] The same type appeared in South America in the aboriginal Caribs and Tupis dwelling along the southern tributaries