The Gradient Boundary
Texas courts have adopted the “gradient boundary” as the usual dividing line between public ownership of a stream’s bed and lower bank area, and private ownership of the higher bank area and the uplands beyond. Thus, there is generally no question as to the public’s right to use the bank area up to the gradient boundary. Sometimes called the “mean” gradient boundary, it is located midway between the lower level of the flowing water that just reaches the so-called “cut bank,” and the higher level of the flowing water that just does not overtop the cut bank. The cut bank is located at the outer edge of a stream’s bed, separating the bed from the adjacent upland and confining the waters to a definite channel. Surveying the gradient boundary is a complex task performable only by specially trained persons.
Judicial Description of the Gradient Boundary
The gradient boundary concept was developed in a U.S. Supreme Court case involving the boundary between Oklahoma and Texas along the south bank of the Red River. After considering the terms of an 1819 treaty between the United States and Spain, the Supreme Court concluded:
Upon the authority of these cases, and upon principle as well, we hold that the bank intended by the treaty provision is the water washed and relatively permanent elevation or acclivity at the outer line of the river bed which separates the bed from the adjacent upland, whether valley or hill, and serves to confine the waters within the bed and to preserve the course of the river, and that the boundary intended is on and along the bank at the average or mean level attained by the waters in the periods when they reach and wash the bank without overflowing it. When we speak of the bed we include all of the area which is kept practically bare of vegetation by the wash of the waters of the river from year to year in their onward course, although parts of it are left dry for months at a time; and we exclude the lateral valleys, which have the characteristics of relatively fast land and usually are covered by upland grasses and vegetation, although temporarily overflowed in exceptional instances when the river is at flood.
The conclusion that the boundary intended is on and along the bank and not at low water mark or any other point within the river bed has full confirmation in available historical data respecting the negotiations which attended the framing and signing of the treaty.
Oklahoma v. Texas, 260 U.S. 606, 631 32, 43 S.Ct. 221, 225, 67 L.Ed.428 (1923).
In Motl v. Boyd, 116 Tex. 82, 286 S.W. 458, 467 (1926), the Texas Supreme Court used language much like that used by the U.S. Supreme Court to describe the bed and the bank of a stream:
A water course, river, or stream consists of a bed, banks, and a stream of water. ... The bed of a stream is that portion of its soil which is alternatively covered and left bare as there may be an increase or diminution in the supply of water, and which is adequate to contain it at its average and mean stage during an entire year, without reference to the extra freshets of the winter or spring or the extreme drouths of the summer or autumn. ... The banks of a stream or river are the water washed and relatively permanent elevations or acclivities at the outer lines of the river bed which separate the bed from the adjacent upland, whether valley or hill, and served to confine the waters within the bed and preserve the course of the river when they rise to the highest point at which they are still confined to a definite channel.
In a 1935 case, the Texas Supreme Court endorsed the gradient boundary concept, stating:
The boundary line is a gradient of the flowing water in the river. It is located midway between the lower level of the flowing water that just reaches the cut bank, and the higher level of it that just does not overtop the cut bank.
Diversion Lake Club v. Heath, 126 Tex. 129, 86 S.W.2d 441, 447 (1935).