CHAPTER III.. The Work of Running Water.

THE WORK OF RUNNING WATER.

+Rain and River Erosion+ 57 Subaërial Erosion without Valleys 58 The Development of Valleys 63 By the growth of gullies, 63. Limits of growth, 67. The permanent stream, 70. Other modes of valley development, 73. Structural valleys, 77. The courses of valleys, 77. The development of tributaries, 78.

+A Cycle of Erosion. Its Stages+ 80

+General Characteristics of Topography Developed by River Erosion+ 92

+Special Features Resulting from Special Conditions of Erosion+ 92 Bad-land topography, 93. Special forms of valleys; canyons, 94.

+The Struggle for Existence Among Valleys and Streams+ 100 Piracy, 103.

+Rate of Degradation+ 105 Material in solution in river water, 107.

+Economic Considerations+ 108

+Analysis of Erosion+ 110 Weathering. 110 Transportation 115 Transporting power and velocity, 115. How sediment is carried, 116. Corrasion 119 Abrasion, 119. Solution, 122.

+Conditions Affecting the Rate of Erosion+ 123 The Influence of Declivity 123 The Influence of Rock 124 Physical constitution, 124. Chemical composition, 124. Structure, 125. The Influence of Climate 127

+Effects of Unequal Hardness+ 132 Rapids and falls, 132. Rock terraces, 140. Narrows, 141. Other effects on topography, 142. Adjustment of streams to rock structures, 146.

+Influence of Joints and Folds+ 150 Joints, 150. Folds, 154.

+Effect of Changes of Level+ 161 Rise, 161. Sinking, 170. Differential movement, warping, 171.

+The Aggradational Work of Running Water+ 177 Principles involved, 177. The Deposits 181 Types, 181. Alluvial fans and cones, 181. Ill-defined alluvium, 183. Alluvial plains, 184. Flood-plains due to alluviation, 186. Flood-plains due to obstructions, 188. Levees, 188. Flood-plain meanders, cut-and-fill, 190. Scour-and-fill, 194. Materials of the flood-plain, 196. Topography of the flood-plain, 196. Topographic adjustment of tributaries, 197. River-lakes, 198. Deltas, 198. Delta lakes, 204.

+Stream Terraces+ 204 Due to inequalities of hardness, 204. Normal flood-plain terraces, 205. Flood-plain terraces due to other causes, 208. Discontinuity of terraces, 209. Termini of terraces, 210.