Planning and location engineers should be conscious of fluvial geomorphology and request the services of hydraulics engineers to quantify natural changes and changes that may occur as a result of stream encroachments, crossings, or channel modifications.

Fluvial geomorphology and river mechanics are not new subjects; however, methods of quantifying the interrelation of variables are relatively recent developments. The theories and knowledge available today make it possible to estimate and predict various reactions to changes and, more importantly, to establish thresholds for tolerance to change.

Streams have inherent dynamic qualities by which changes continually occur in the stream position and shape. Changes may be slow or rapid, but all streams are subjected to forces that cause changes to occur. In these streams, banks erode, sediments are deposited, and islands and side channels form and disappear in time. The banks and adjacent floodplains usually contain a large proportion of sand, even though the surface strata may consist of silt and clay; thus, the banks erode and cave with relative ease.

Most alluvial channels exhibit a natural instability that results in continuous shifting of the stream through erosion and deposition at bends, formation and destruction of islands, development of oxbow lakes, and formation of braided channel sections.

The degree of channel instability varies with hydrologic events, bank and bed instability, type and extent of vegetation on the banks, and floodplain use.

The identification of these characteristics and understanding of the relationship of the actions and reactions of forces tending to effect change enables the design engineer to estimate the rates of change and evaluate potential upstream and downstream effects of natural change and proposed local channel modifications.

The potential response of the stream to natural and proposed changes may be quantified with the basic principles of river mechanics. The design engineer must understand and use these principles to minimize the potential effect of these dynamic systems on highways and the adverse effects of highways on stream systems.

Non-alluvial channels have highly developed meanders in solid rock valleys and may be degrading their beds. An example of such a stream is the Guadalupe River as it passes through the Edwards Aquifer recharge zone. Many mountain streams are classified as non-alluvial, and in these cases the design engineer may perform a hydraulic analysis utilizing rigid boundary theory.