The profiles here represented are derived from various sources and differ from one another greatly in accuracy. Many of them are drawn from the annual reports of the Chief of Engineers, U.S.A., under which are included the reports of the Mississippi and Missouri River commissions. The heights thus obtained are those of the level of water in the rivers at certain stages, and may be regarded as of great accuracy. Others have been obtained from railroad profiles, being the level of the rivers at points where the railroads touch or cross them. Still others have been taken from the atlas sheets of the United States Geological Survey and from other maps. In most such cases the points at which the contours cross the rivers upon the maps have been taken. These again differ in point of accuracy with the means adopted for the location of the contours. Where the spirit level was used in locating contours it may be assumed that the determinations are fairly good, but where the barometer was used the probable error may be of considerable magnitude. Such elevations are, however, mainly in the mountainous parts of the country, where the fall of the streams is great, and where, therefore, errors of considerable magnitude may be tolerated, as affecting but little the form of the profile.
