River features; hydrological cycle, aquifer

Morphological features of river channels

Erosional work of a river is done by hydraulic action, abrasion, attrition and solution. Features:

• V-shaped valley: formed by vertical (downward) cutting in the youth stage, in the hills.
• Gorge and canyon: deep, narrow valley in hard rock, e.g. the Kali Gandaki gorge.
• Waterfall and rapids: form where hard rock lies over soft rock; a plunge pool forms at the base.
• Potholes: circular holes drilled in the bed by swirling pebbles.
• River terraces: step-like old floodplain levels left after the river cuts down again.
• Meanders and ox-bow lakes: formed by lateral (side) cutting in the old stage.
• Interlocking spurs: ridges of the valley sides that project alternately into the winding valley of a young river.

Depositional work occurs when the velocity and carrying capacity fall (on gentle slopes, at the river mouth, and on the floodplain during floods). Features:

• Alluvial fan: cone-shaped deposit where a mountain stream meets a plain, e.g. the Bhabar zone at the foot of the Siwaliks.
• Floodplain: broad, flat valley floor built by flood deposits.
• Natural levees: low ridges of sand and coarse silt built along the banks during floods.
• Braid bars and point bars: sand and gravel bars in the channel (braid bars) and on the inner bank of bends (point bars).
• Delta: fan-shaped deposit at the river mouth, e.g. the Ganges-Brahmaputra delta.
• Ox-bow lake and meander scars: cut-off bends.

Definitions

• Hydrological cycle: the continuous circulation of water between the oceans, atmosphere and land: evaporation, condensation, precipitation, runoff, infiltration and groundwater flow back to the sea.
• Porosity ($n$): ratio of the volume of voids to the total volume of the rock or soil, $n = V_v/V$. It measures the water storage capacity.
• Permeability: the ability of a material to transmit fluid through its interconnected pores. Gravel is highly permeable; clay is almost impermeable.
• Aquifer: a saturated rock or soil formation that stores and yields useful amounts of water, e.g. sand, gravel, fractured rock.
• Aquiclude: a saturated formation that stores water but cannot transmit it in useful amounts, e.g. clay, shale.
• Piezometric level: the height to which water rises in a well drilled into a confined aquifer; the imaginary surface joining these levels is the piezometric (potentiometric) surface.