Kotły wirowe w krajobrazie dolin rzecznych zachodniej Hiszpanii

Spectacular bed forms, eroded by flood flows, are a feature of the bare rock terraces and channels of some rivers cut into granitic rock in the districts of Salamanca, Cceras and Badajoz, W Spain. The bed forms comprise a range of pothole types which individually represent various stages in the evolution of mature potholes. Pothole evolution reflects the hydrodynamics of the erosion, by high velocity flood flows, of bedrock. They evolve in time and in concert with the evolution of the landscape. Initial shallow depressions deepen gradually into cylindrical forms as vertical erosion increases. As time passes, lateral erosion increases and leads to wider cylindrical forms which evolve into asymmetrical and bulbous forms. Second-order scouring leads to the development of minor decorative potholes, tunnels and niche caves in the large, mature potholes. Data on pothole geometry should complement the findings of other geomorphological studies on individual rivers and on regions. The pattern of pothole maturity in a particular river valley can be used as a measure of the "erosionalmaturity" of the valley in the following way. All other things being equal, a pothole pattern involving many immature forms should characterize "young" stages in the erosion of a bedrock surface and, conversely, a preponderance of mature pothole forms should be a feature of "older" stages. Knowing the proportions of the various pothole types in the three areas studied, this simple rule allows the following conclusions. (a) The Salor River valley represents a "young" erosional stage; immature pothole forms predominate and the largest potholes are sited close to the present river channel. (b) The present valley of the Tormes River displays a similar relatively "young" evolutionary stage. The positioning of the large mature potholes above the present river level suggests that the water level in this river was, for a considerable time in the past, higher than it is today. (c) The Jerte River valley represents, essentially, an "old" evolutionary stage overprinted by "younger" erosional event(s). However, in this case, the local geological structure - a series of parallel faults - played an important role in the development of some large sumps (F-type potholes) at the bases of waterfalls. In the evolutionary history of a river channel, pothole formation is rightly recognized as a most potent method of down cutting. At any given moment, however, they are a morphological detail. Other than recognizing the abrasion they reflect, they may tend to be ignored. However, their geometries may allow a point in the history of a river, i.e., its present maturity, to be precisely defined. In addition, the spatial locations of potholes in river valleys reveal past and seasonal water levels. This study in W Spain suggests that a more complete and quantitative knowledge of pothole development in the rivers of the region could profitably complement and refine the findings of other methods of geomorphological measurement.