Lecture on Surface Water and Rivers

Today's topic: Surface water, with a primary focus on rivers which exhibit channelized flow.
Importance of understanding rivers in shaping the Earth’s landscape through erosion and sediment deposition.
Total estimated lecture duration: 1.5 hours.

Definition: Water constrained between river banks (sides) and the riverbed (bottom).
Difference from sheet flow (e.g. water pooled in streets after rainfall).

Freshwater constitutes less than 3% of Earth's total water budget; majority is seawater.
Most freshwater is stored in glaciers (melting poses threats to water supply).
Freshwater is also found in groundwater, which is the largest store of fresh water, located underground.

Definition: Erosion process by which a river lowers its own profile by cutting into solid rock.
Forms Slot Canyons: Occur when rivers erode through solid rock, creating deep canyon walls.
Importance of solid rock types: Strong, solid rock leads to slot canyons, while weaker rocks lead to other river formations.

Occurrence: When downcutting happens in loose or weak materials, creating a V shape rather than a slot canyon.
Erosion: Downcutting involves mass movement (removal) of materials termed as erosion, not to be confused with weathering (just breaking materials into smaller pieces).
Controlling factor:
- Angle of Repose: Maximum stable angle a slope can have before materials start falling into the river, affecting its shape.

Changes observed as one moves from the headwater region to the river delta:
1. Decrease in downcutting as the river approaches base level (the lowest point a river can erode).
2. Change in water volume and slope gradient.
Segments A, B, C: Represent the headwater region (A with steep gradient), mid-section (B with moderate gradient), and delta region (C with gentle slope).
Gradient (Slope) Decreases:
- Example: Segment A (15m drop), Segment B (8m drop), Segment C (4m drop).

Discharge (Q): The volume of water passing a point in the river over time.
Equation: Q = A \times V
- Where A = area of the water flow, V = velocity of the water.
Discharge typically increases downstream due to the addition of water from tributaries.

Relationship between area and velocity:
- Given a constant discharge, if area decreases, velocity must increase to maintain the same discharge.
- Conversely, if area increases, velocity decreases.
Example: Using a garden hose to illustrate how reducing the area (thumb over the nozzle) increases the water speed.

Fast-moving water erodes and picks up materials; slow-moving water deposits what it carries.
Consequences of channelized flow: Increased erosion in fast-moving areas may undermine the riverbank or structures like levees.

Alluvial Fan: Formed where a river opens up into a wider area, leading to sediment deposition.
Deltas: Form at the river's mouth when entering an ocean, leading to diverse sediment types like sand and silt.

Straight Channels:
- Characteristics: Fastest flow is in the center; ideal for canoeing downstream.
Meandering Rivers:
- Formed with bends (meanders); the outer banks (cut banks) experience erosion, while inner banks (point bars) see deposition.
- Erosion vs. deposition leads to fertile floodplains, creating suitable farming lands.
Braided Streams:
- Characteristics: Form in areas with high sediment loads; channels crisscross like braids.
- Common locations: Semi-arid regions, outwash plains near glaciers, and river deltas where sediment deposition occurs.

Definition: Area adjacent to the river that is prone to flooding.
Fertility: Floodplains often have rich, nutrient-filled deposits that are beneficial for agriculture.

Levees: Structures built to raise the banks of a river to prevent flooding; can be made from earth or concrete.
Dams: Control river discharge; however, can hinder natural sediment flow and affect ecosystems adversely.

Hydrographs: Plots showing river discharge over time, used to understand flood patterns.
Recurrence Interval (RI): Time period between floods of a specific size (larger floods occur less frequently).
Hydrograph interpretation essential for risk assessment in flood zones.

As rivers progress from headwaters to deltas, notable trends include:
- Decreased downcutting, increased discharge and drainage basin size, and decreased slope gradient.