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Summary. The diverse formulae used to measure the capicity of transport of alluvium in watercourses provide incomplete information on the part played by the width /depth ratio of these channels. Neither do they provide sufficient information as to whether the concentration of flow is a factor sufficient in itself to explain the progressive downstream lessening of the slope of a stream profile. We should draw a distinction between the phenomena of « useful » friction - ie. a dynamic contact accompanied by transport of alluvium, and the « sterile » rubbing of molecules of water against each other.

Summary. The river Cady, a tributary of the Tet, rises on the north face of the Canigou and flows through the basin of Vernet-les-Bains. Spring and autumn rainfall are both intense and abundant in the region, and maximum floodwater appears at that time. The transportation of sediment has been studied in the middle part of the Cody, using mostly radioactive tracers. The river bed is not very twisty and well-graded (average width 10 metres), but slopes fairly steeply (43 %c), and is littered with rocks and boulders. The critical phase of deplacement of sediment starts at about 6 m ? per sec. The alluvium moves over the entire width of the bed and is deposited either in the axis of flow or on a convex bank. Transport for q floods (maxima 10.5 and 36.5 ma per sec.) is respectively 7 and 34.6 tons per metre of width. A q flood of max. 10 m3 per sec. can carry away blocks of 20 cm. in diameter, and a q flood of max. 36.5 m3 per sec. larger blocks of 30-40 cm. diam. These values for the beginning of the critical phase provoking transport are in agreement with others calculated by the means of formulae. Values for the transport itself, however, are lower than those calculated by the Meyer-Peters formula (qs = k (t-t0)3/2 where k = 1,29). These results may explain renewed erosion in the bed of the Cady, amongst the chaos of pebbles, rocks and boulders, and may be used to interpret forms of accumulation of the larger material, also the evolution of these forms in intermontane basins and in piemont an foothill. floodwater appears at that time. The transportation of sediment has benn studied in the middle part of the Cody, using mostly radioactive tracers. The river bed is not very twisty and well- graded (average width 10 metres), but slopes fairly steeply (43 %c), and is littered with rocks and boulders. The critical phase of déplacement of sediment starts at about 6 m ? per sec. The alluvium moves over the entire width of the bed and is deposited either in the axis of flow or on a convex bank. Transport for q floods (maxima 10.5 and 36.5 ma per sec.) is respectively 7 and 34.6 tons per metre of width. A q\ flood of max. 10 m3 per sec. can carry away blocks of 20 cm. in diameter, and a q flood of max. 36.5 m3 per sec. larger blocks of 30-40 cm. diam. These values for the beginning of the critical phase provoking transport are in agreement with others calculated by the means of formulae. Values for the transport itself, however, are lower than those calculated by the Meyer-Peters formula (qs = k (t-t0)3/2 where k = 1,29). These results may explain renewed erosion in the bed of the Cady, amongst the chaos of pebbles, rocks and boulders, and may be used to interpret forms of accumulation of the larger material, also the evolution of these forms in intermontane basins and in piemont an foothill.

Summary. In studying the forms and evolution of river beds, radioactive tracers have proved useful in complementing other methods of observation, and have also been used as arbitrators between two opposing hypotheses on evolution. Tracers help in discovering if a river is in fact eroding when the bed reveals no trace of eroding action. More fully, they may provide the depth of the sediment transported, hence an estimate of the total transport. When the necessary precautions are taken these tracers can be a considerable aid to advance in the study of certain geormorphological problems, such as the rate of vertical erosion of the bedrock, the evolution of long profiles, and profile of equilibrium.

Summary. The drainage basin of the Rivers Semouse, Augronne and Combeaute is arranged around the geological structure of the region. Alluvium at the mouths of the Vosges valleys is a result of glacial and fluvial erosion, but the manner of deposit also provides evidence as to the evolution of the tectonics of the region.