Hydrology and Landscapes

Introduction to Surface Water and the Hydrologic Cycle

  • Running Water's Role
    • Water flow over Earth's surface is vital in the hydrologic cycle.
    • It significantly influences the topography and landscape of regions.
    • Example: The Grand Canyon illustrates how water can shape and create landforms.

Water Flow and Landscape Formation

  • Deltas Formation
    • Running water in rivers and streams can form deltas (e.g., mouth of the Mississippi River).
    • A significant portion of Louisiana consists of land formed by deposition at the Mississippi River Delta.

Focus Areas of Lecture

  • The lecture emphasizes:
    1. Stream flow characteristics
    2. Sediment transport processes

Stream Flow Characteristics

  • Key Factors of Stream Flow

    1. Type of Flow
    • Describes the relational movement of water molecules.
    • Laminar Flow: Water molecules move parallel without interaction (layered flow).
    • Turbulent Flow: In fast waters or over rough surfaces, molecules interact, leading to eddies and whirlpools.
      • Critical for sediment transport as it keeps more sediment in suspension.
    1. Flow Velocity

    • Speed of water flow; influenced by multiple factors such as:

      • Slope/gradient of the stream.
      • Roughness of the stream bed.
      • Shape of the stream channel.

    • Steeper gradients result in faster flow velocities.

    • Rough channel beds create friction, decreasing velocity; smooth beds increase velocity.

    • Channel shapes that minimize friction enhance flow velocity.

    1. Stream Discharge
    • Total volume of water flowing past a location within a specific time frame, measured in cubic meters per second (m³/s).
    • Variability in discharge occurs over time (e.g., droughts, precipitation variations).
    • Discharge generally increases downstream due to tributary additions and groundwater inputs.

Sediment Transport by Flowing Water

  • Types of Sediment Transport

    • Suspended Load: Small particles (clay, mud) held in water suspension; contributes to water color (e.g., Yellow River's muddy appearance).
      • Increased velocity during floods boosts suspended load capacity.
      • High sediment concentration can increase water's viscosity, analogous to molasses.

  • Bed Load: Larger sediment too heavy for suspension; moves along the stream bed through:

    1. Saltation: Particles bounce along the bottom.
    2. Traction: Particles slide or roll along the bottom; significant for stream bed erosion.

  • Dissolved Load: Ions dissolved in water from chemical weathering (e.g., calcium, sodium, potassium).

    • Defines ocean salinity as rivers meet the ocean and deposit their dissolved loads.

Erosional Processes in River Systems

  • Erosion Mechanisms

    • Running water as a powerful erosional agent:
    • Transports sediment through erosion of stream channels and surrounding landscapes.
    • Movement along stream beds leads to abrasion and creates valleys and canyons through a process called downcutting.

  • Valley Formation

    • In humid areas: downcutting leads to valleys, with concurrent weathering on slopes.
    • In arid areas: downcutting usually results in canyons due to less rock weathering.
    • Features:
    • V-shaped valleys indicate active downcutting.
    • Decreased gradient reduces downcutting, leading to wider river valleys and flood plains.

  • Meandering Streams

    • Streams on lower gradients tend to meander, creating curves.
    • Erosion and Deposition Dynamics:
    • Water flows faster on the outer bend of meanders (creating erosion) and slower on the inner bend (leading to deposition).
    • Erosion on the outer bend can create cut banks while deposition on the inner curve forms point bars.

  • Incised Meanders

    • Result from tectonic movements or sea level changes, causing streams to cut into bedrock, creating stable meander shapes.
    • Example: Features found in Canyonlands National Park, Utah.

Depositional Features in River Systems

  • Sediment Deposits

    • Rivers can deposit sediments when they slow down, leading to the formation of features such as:
    • Channel Bars: Deposits creating mid-channel features due to sediment release when stream velocity drops.
    • Point Bars: Formed on the inside of meanders due to sediment deposition during slower flow.

  • Braided Streams

    • Result from high sediment loads, where multiple channels form as seen in glacial areas or close to mountains.

The Dynamics of Floodplains

  • Floodplain Characteristics
    • Flooding leads to deposition events, enriching floodplains with nutrients.
    • Example: The Fertile Crescent resulting from deposition in river valleys (e.g., Tigris River).
    • Natural levees form along riverbanks during floods by depositing coarser sediments closest to the river.

River Mouths and Delta Formation

  • Meeting with Oceans

    • Rivers ultimately meet the ocean, where sediment deposition occurs as velocities drop significantly.
    • Coarse sediments settle near river mouths, whereas finer sediments may be carried further out.

  • Delta Progradation

    • If sediment deposition exceeds erosion, deltas grow over time, creating distinct layers with coarser sediments nearer the river and finer sediments farther out.

  • Types of Deltas

    • Bird's Foot Delta: E.g., Mississippi River Delta (stream-dominated).
    • Wave-Dominated: E.g., Nile River Delta, characterized by barrier islands.
    • Tide-Dominated: E.g., Ganges Brahmaputra Delta, formed by tidal actions affecting sediment transport and deposition.

Conclusion

  • The processes discussed are crucial for understanding the significant role of running water in both shaping landscapes and transporting sediments across diverse environments.