River erosion landforms

Valleys: from rills and gullies grow different valley shapes: V-shaped valleys, gorges and canyons.

• A gorge is a deep valley with very steep to straight sides, about as wide at the top as at the bottom. Gorges form in hard rocks. (The book's example is the Kaveri near Hogenakkal, Tamil Nadu.)
• A canyon has steep, step-like side slopes and is wider at the top than the bottom; it may be as deep as a gorge, and is a variant of one. Canyons form in horizontally bedded sedimentary rocks. (Example: the Colorado river, USA.)

Potholes and plunge pools:

• On the rocky beds of hill streams, round hollows called potholes form. Once a small hollow exists, pebbles and boulders collect in it and are spun round by the water, drilling it deeper and wider. Neighbouring potholes join, deepening the valley.
• At the foot of waterfalls, the force of falling water and spinning boulders make large, deep holes called plunge pools.

Incised (entrenched) meanders: fast streams on steep slopes erode mostly downward; streams on gentle slopes erode sideways and meander. But deep, wide meanders can also be found cut into hard rock: these are incised or entrenched meanders.

River terraces mark old valley floor or floodplain levels. They may be bare rock or covered with stream deposits. They are basically products of erosion: the river cuts down into its own old floodplain, leaving the former floor standing above. Several terraces at different heights show former river-bed levels. Terraces at the same height on both sides are paired terraces.

Example (new): a narrow slot in granite with walls almost straight up and down, barely wider at the top than at the river, is a gorge; a wide valley with ledges like a giant staircase in layered sandstone is a canyon.