Comprehensive Study Notes: River Systems and Processes

Introduction to Hydrological Systems

• Global Water Distribution: Roughly $70\%$ of the Earth’s surface is covered with water. Most of this water is contained in oceans and seas.
• The Hydrological Cycle (Water Cycle): This is the constant movement of water around the planet from one point to another, occurring on, below, or above the Earth's surface and the atmosphere.
• The Drainage Basin System: The hydrological cycle operates as a system consisting of inputs, storage, flows (transfers), and outputs.

Processes of the Water Cycle

• Inputs:

  • Precipitation: Any source of moisture reaching the ground. Examples include rain, snow, and frost.

• Storage:

  • Interception: This occurs when water is prevented from reaching the ground surface by vegetation, such as trees or grass.
  • Surface Storage: Water held on the ground surface, such as in puddles or ponds.
  • Soil Moisture: Water that is held within the soil layer.
  • Groundwater: Water stored within the rock layers.
  • Aquifer: A saturated rock layer that forms an underground water reservoir.
  • Water Table: The current upper level of saturated rock or soil where no more water can be absorbed.

• Flows (Transfers):

  • Infiltration: The process of water sinking into the soil or rock from the ground surface.
  • Percolation: Water seeping deeper below the soil surface into the rock layers.
  • Surface Run-off (Overland Flow): Water flowing across the top of the ground.
  • Throughflow: Water flowing through the soil layer parallel to the ground surface.
  • Groundwater Flow: Water flowing through the rock layer parallel to the surface.
  • Spring: Water emerging onto the ground surface from an underground source.

• Outputs:

  • Evaporation: Water lost to the atmosphere from the ground or vegetation surfaces.
  • Transpiration: Water lost to the atmosphere through the pores in vegetation.
  • River Discharge: Water carried by a river to a lake or the sea.

Fundamental River Definitions

• River: A fairly large body of water which flows in a channel and through which excess water is drained away from the land.
• Source: The place where a river begins. Common sources include glaciers, springs, or lakes.
• Mouth: The place where a river ends its journey, typically in an ocean, sea, or lake.
• Tributary: A smaller stream or river that flows into a larger main river.
• Confluence: The point where two rivers or streams meet.
• Channel: The physical confine of a river consisting of a bed and banks.
• Drainage Basin: The entire area of land drained by a river and its tributaries.
• Watershed: The topographic barrier or boundary that divides one drainage basin from another. It is generally represented by high land.

Drainage Patterns

Rivers form distinctive patterns influenced by the topography (terrain), the gradient of the land, and the underlying geology (the presence of hard or soft rocks).

• Dendritic Drainage Pattern:

  • Description: This is the most common pattern in the Caribbean and resembles a tree; the main river looks like the trunk, tributaries resemble branches, and small streams are like twigs.
  • Development: It develops in areas where the rock type is uniform and the land is gently sloping. It typically occurs where no major geological factors interfere with the flow.

• Radial Drainage Pattern:

  • Description: This occurs when rivers flow away from a central area of high land, like spokes on a wheel.
  • Example: Rivers flowing down the slopes of volcanoes or domes. In Montserrat, the drainage is primarily radial as rivers flow away from the Soufriere Hills/volcanic centre.

• Trellis Drainage Pattern:

  • Description: This pattern occurs when tributaries join the main river at near-right angles.
  • Development: It occurs in areas where there are alternating bands of resistant (hard) and less resistant (soft) rock at right angles to the direction of the slope. The main river follows the slope across the rock bands, while tributaries follow the softer rock bands to join the main channel at right angles.

The Work of the River

A river acts as a "sculptor," powerful enough to shape the landscape through three primary functions: Erosion, Transportation, and Deposition.

• River Energy: A river's energy is used to transport load, erode its bed/banks, and overcome friction from the rough bed. Energy is determined by:
  • The height above sea level.
  • The volume of water.
  • The speed (velocity) of the water.

Processes of River Erosion

Erosion is the wearing away of the river bed and banks. It occurs in four specific ways:

• Hydraulic Action: The force of the water splashing against the river banks, forcing its way into cracks and openings. This pressure loosens material along the bed and banks.
• Corrasion (Abrasion): The river uses its load (stones and grit) to grind against the bed and banks, sand-papering the surface and loosening material.
• Attrition: Material carried by the river (rocks and stones) collide with each other. Over time, they break down into smaller, smoother, and more rounded particles.
• Solution: River water dissolves soluble minerals in rocks and soil (e.g., salt or calcium carbonate) as it flows over them.

Directions of Erosion:

• Vertical Erosion: The river cuts downwards into its bed, deepening the valley. This is most dominant in the upper course.
• Lateral Erosion: The river erodes its banks (sides), widening the valley. This is dominant in the middle course.

River Transportation

Transportation is the process by which a river carries its load. This happens through four methods:

• Traction: The heaviest material (large rocks/boulders) is rolled along the riverbed.
• Saltation: Smaller stones and pebbles are bounced along the riverbed in a leap-frog motion.
• Suspension: Tiny particles like silt and clay are carried within the water without touching the bed. During heavy rain, this silt can turn river water brown.
• Solution: Dissolved minerals are carried downstream in the water.

Deposition

Deposition occurs when a river drops all or part of its load. This happens when the river loses energy due to:

• A decrease in the speed (velocity) of the water.
• A decrease in the volume of water in the channel.

Deposition Rule: As energy levels drop, the heaviest materials (large rocks) are deposited first, while lighter materials (fine silt) are carried further downstream.

Stages of a River

A river valley is divided into three stages, each with distinct characteristics and landforms.

The Upper Course (Upper Valley)

• Gradient: Steep.
• Main Activities: Vertical (downward) erosion; Hydraulic action, abrasion, and attrition are dominant. High energy allows for the transport of large, angular load via traction and saltation.
• Features: Source, V-shaped valleys, interlocking spurs, waterfalls, rapids, and tributaries.
• Cross Profile: Narrow, V-shaped valley with steep walls; shallow channel.

The Middle Course (Middle Reaches)

• Gradient: Gentle.
• Main Activities: Vertical erosion decreases; lateral (sideways) erosion starts along with deposition. Transportation is primarily through suspension.
• Features: Meanders, ox-bow lakes, river cliffs, and slip-off slopes. Load becomes smaller and less angular.
• Cross Profile: Wider valley with sloping walls; channel is deeper and wider.

The Lower Course (Lower Reaches)

• Gradient: Very low (flat).
• Main Activities: Deposition is more important than erosion. The river carries a large volume of fine, very rounded load.
• Features: Deltas, braiding, flood plains, levees, and the mouth. The channel may be tidal.
• Cross Profile: Broad valley with a wide, swampy flood plain; the channel is at its widest and deepest.

Long and Cross Profiles

• Long Profile: The change in gradient with distance from source to mouth. It starts steep and becomes a concave curve as it approaches sea level.
• Elevations: The profile typically starts at higher elevations (e.g., $500\,m$) and descends to sea level ($0\,m$) or slightly below ($-100\,m$) at the mouth.

Case Study: Wag Water River, Jamaica

• Upper Course: Located near Stony Hill, St. Andrew.
• Middle Course: Located through St. Mary.
• Lower Course (Mouth): Located at Annotto Bay, St. Mary.