Study Notes on River Environments

Water moves between stores through various flows:
  - Evaporation: The process where water is converted from a liquid to a gas (water vapor) due to solar energy, primarily from seas, ponds, and lakes.
  - Transpiration: The uptake of liquid water by plants from the soil, which releases vapor into the atmosphere.
  - Evapotranspiration: The combined loss of moisture from the soil and water bodies through evaporation and transpiration.
  - Condensation: Water vapor cools in the atmosphere and transforms into liquid water droplets, forming clouds.
  - Precipitation: Water transferred from the atmosphere back to the land or sea in forms such as rain, snow, or hail.
  - Overland Flow: The process where precipitation that hits the ground moves due to gravity, entering streams, rivers, or lakes—also termed runoff.
  - Infiltration and Percolation: Water moves downward through soil and rock to reach aquifers or groundwater storage.
  - Throughflow: The movement of water horizontally within the soil before it reaches a stream or river.
  - Groundwater Flow: The underground transfer of water in aquifers supplying rivers, lakes, and seas.
All these flows form a cycle involving three main stores: atmosphere, land, and sea. Water can return to the atmosphere directly via transpiration or evaporation without reaching the sea.

The main stores of water in the hydrological cycle include:
  - Atmosphere: Contains water vapor or tiny droplets in clouds.
  - Land: Water exists in rivers, lakes, reservoirs (surface) and aquifers (groundwater). Water can be in liquid, solid (ice), or gas (vapor) form.
  - Sea: Contains over 95% of Earth's water, primarily in liquid form, with some existing as ice in icebergs.
The relative amounts of water in these stores can change due to solar energy variations, affecting evaporation and melting ice sheets.

Every river has its drainage basin or catchment area, where water moves similarly to a smaller hydrological cycle, but is an open system with external inputs and outputs. The drainage basin varies over time and the hydrological cycle is fixed.

Inputs:
  - Energy from the Sun
  - Precipitation from moisture outside the basin
  - Water from tributary drainage basins (not always visible)
Outputs:
- River discharge
- Water from the basin that is subject to evaporation and transpiration before precipitation occurs in another basin.

The watershed is the dividing line between neighboring drainage basins, following the hilltops.
Rivers originate in higher basin areas near the watershed where most precipitation falls. Smaller streams known as tributaries join at confluences to the river, while rivers flow into the sea at their mouths or estuaries.

Three broad types:
  - Basins that transport water directly to the sea.
  - Parts of larger drainage basins (e.g., River Negro contributing to the Amazon).
  - Basins that lead into inland seas or lakes, like the Caspian Sea or Lake Victoria.

The drainage network consists of surface and subterranean channels collecting precipitation. The density of a basin's network refers to the number of channels per unit area, which can change drastically during flood events.
Each drainage basin has unique characteristics influencing water movement, including:
- Size, shape, rock type, relief, and land use.
Factors affecting runoff:
- Woodland vs. cleared land for agriculture: Woodland retains water and slows runoff, while agriculture speeds it up. Urban areas with hard surfaces also hasten runoff.

Understanding how rapidly rainfall reaches the drainage network is crucial for assessing flooding risks.
Discharge: The volume of water flowing past a specific point, measured in cubic meters per second (cumecs).
River regimes reflect the natural variations in discharge over time, influenced by seasonal rainfall patterns. Hydrographs illustrate these variations.

Average Monthly Discharge of River Ganges:
- High discharge between June and October coinciding with the monsoon (total rainfall exceeding 2750 mm). (Figure 1.6)
River Thames Hydrograph (2001-02): (Figure 1.7)
- Discharge peaks in winter months, indicating jagged discharge related to short bursts of rainfall.

A storm hydrograph maps changes in river discharge after rainfall. Key concepts include:
- Lag Time: The delay between peak rainfall and peak discharge, shorter lag leads to steeper discharge rises and higher flood risk.
The graph can indicate flood risk levels when plotted with typical discharge ranges.

Rivers shape landscapes through erosion, transport, and deposition alongside weathering and mass movement processes.

Physical Weathering: Breakdown of rocks through temperature changes and moisture.
Chemical Weathering: Decay of rocks via acid rain infiltration.
Biological Weathering: Plant roots infiltrating rocks leading to disintegration.

Hydraulic Action: Water forces dislodge materials from the riverbank and bed.
Abrasion: Material carried by the river wears away the channel sides and floor.
Corrosion (Solution): Water dissolves minerals in river channel rocks.
Attrition occurs when sediment particles collide and become smaller.

Occurs when river energy decreases, often at lake/seas or reduced gradient areas. Most deposition occurs during low-energy stages in the journey toward the sea or within the floodplain.

The Long Profile of a river shows how its physical characteristics vary from source to mouth. Generally, it shows a concave shape, being steeper in upland areas and gentler in lowlands.
Downstream Changes: (Figure 1.11)
  - Load Particle Size: Finer and more rounded downstream.
  - Discharge: Increases steadily downstream.
  - Channel Characteristics: Narrow and shallow at the source, becoming wider and deeper downstream.