Topic 1.1 Rivers and Drainage Basins

Hydrological Cycle and Process Definitions

  • The water cycle, or hydrological cicle, involves water movement between the atmosphere, land, and sea.

  • Inputs and Outputs: Precipitation (rain, sleet, snow) and Evapotranspiration (combined evaporation and transpiration from leaves).

  • Surface Processes: Interception (trapping water on leaves), Stemflow (flow down stems), Throughfall (flow through vegetation), and Overland flow (surface runoff when soil is saturated).

  • Subsurface Processes: Infiltration (surface to soil), Infiltration capacity (maximum absorption rate), Throughflow (downslope movement within soil), Percolation (downward through subsoil), and Baseflow (groundwater entering the river channel).

  • Storage and Catchment: Recharge (percolation into groundwater store), Antecedant moisture (pre-existing soil moisture), Porosity (pore capacity), and Permeability (fluid transmission ability).

River Profiles and the Bradshaw Model

  • Long Profile: A line representing the river from its Source (upland start) to its Mouth (sea end).

  • Upper Course: Characterized by steep slopes, large load, and dominant verticle erosion.

  • Middle and Lower Course: Characterized by flatter land, lateral erosion, and finer sediment.

  • Cross Profile: Moves from narrow, shallow, V-shaped valleys in the upper course to wide, deep, and efficient channels in the lower course.

  • The Bradshaw Model: Predicts that downstream flow increases in discharge, channel width, channel depth, and average velocity (m/sm/s). Downstream flow decreases in bedload particle size, channel bed roughness, and gradient.

Fluvial Processes: Erosion, Transportation, and Deposition

  • Erosion types: Hydraulic Action (water force), Abrasion / Corrasion (load scouring bed/banks), Attrition (particles colliding), and Solution / Corrosion (chemical dissolution of rocks like limestone).

  • Transportation types: Traction (dragging large boulders), Saltation (bouncing pebbles), Suspention (carrying light silt/clay), and Solution (dissolved minerals).

  • Deposition: Occurs when velocity and energy decrease, causing the river to drop heavier load first.

River Landforms

  • Upper Course: Potholes (formed by abrasion), Waterfalls (over resistant hard rock and soft rock), Gorges (retreating waterfalls), and Interlocking Spurs.

  • Middle and Lower Course: Meanders (bends shaped by centrifugal force and the thalweg current), River Cliffs (outer bend erosion), and ox-bow lakes (isolated meander remnants).

  • Flood Plains and Levees: Flat land besides rivers and raised embankments formed by repeated overbank deposition.

  • Deltas: Depositional forms at the mouth involving flocculation and distributaries. Types include arcuate (fan-shaped) and bird's foot (e.g., Mississippi river delta, Nile river).

River Hazards, Opportunities, and Management

  • Opportunities: Water Supply for drinking/industry, Transportation for goods and people, and Recreation (fishing, boating, swimming).

  • Hazards: Flooding (property damage), Erosion (wearing away land), and Water-borne diseases.

  • Management: Afforestation (planting trees), building dams, flood walls, Levees, and Channelisation (straightening, widening, or deepening channels).