River morphology; porosity, aquifer, piezometric level

Morphology of river channel

River channel morphology is the form of the channel: its cross-section, long-profile, slope and plan form. By plan form, channels are straight, meandering, braided or anastomosing. In the long profile, a river changes along its length from a steep, narrow, V-shaped upper course (erosion) through a middle course to a gentle, wide lower course with meanders and floodplain (deposition).

River channels are classified by their plan shape (pattern) as:

 Straight:       ----------------------
                 ======================

 Meandering:      ___      ___
                 /   \    /   \
              __/     \__/     \__

 Braided:         ___   ___   ___
                 /   \_/   \_/   \
                 \___/ \___/ \___/

1. Straight: nearly straight, steep, in rocky hills.
2. Meandering: sinuous loops on gentle slopes; outer bank erosion, inner bank deposition.
3. Braided: many channels around bars with heavy bed load, e.g. Koshi in the Terai.
4. Anastomosing: stable multiple channels with vegetated islands.

Definitions

• 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.