Forms by Flowing Water (Fluvial Features)

Abstract

Water is the most precious element on our planet, and therefore rivers also are called “veins of life”, particularly in arid or semi-arid landscapes. Rivers, however, are the forces which form valleys by cutting linear to oscillating depressions into the landscape. Even if the river is no longer present, we can easily identify the valleys as the result of previously flowing water (e.g., by well-rounded and stratified sediments, and the typical geomorphology of valleys). Evidently, to cut valleys in hard rock requires more than water. Debris of different sizes (from silt to boulders) transported by flowing water are the real tools of erosion and are responsible for the incision of valleys. Valley incision, however, needs some time, and valleys therefore may be rather old forms, up to many million years old. Exceptions are gully-like forms in soft rock, which may start to form within a single strong rainfall event. All in all, valleys of different length, depth, or form and shape are amongst the most frequent features in geomorphology, which gives apparent evidence that our Earth is a “water planet” even on the continents. In this chapter we will present examples of valleys in different climates and topographies, which exhibit different conditions of flowing water and sediment transport. The first part deals with flowing water and the incision of valleys, with additional specific features like meandering or anabranching rivers. The second part shows how rivers accumulate sediments in the forms of debris fans or river terraces. River terraces (also fluvial or alluvial terraces) accompany most rivers of the Earth as elongated and level former floodplains above the present one. These terraces are relicts of fluvial sediment accumulation and/or fluvial erosion and give evidence of the long and complex history of a river’s evolution indicated by periods of cut (fluvial incision) and fill (fluvial accumulation). As such, they indicate past changes of water discharge, stream power and sediment input, which may be triggered by climatic changes. In the temperate regions, generally known as areas influenced by strong climatic fluctuations during the last Ice Age, older valleys are filled by river sediments and the successive incision of the rivers into these sediments has formed a series of terraces along many rivers. In addition to climate, tectonics may influence terrace building by forcing the river to incise or deposit their sediments. Another form of fluvial deposition (except of fluvial deltas along the coasts of the Earth’s oceans and along the shores of lakes) is alluvial fans. They usually have the form of cones, starting at the point where rivers abruptly exit steeper and narrow valley sections and enter lowlands with reduced inclination, distributing their debris to all sides in front of a mountain range.