7 rivers lakes and landscapes

Water on Land Surface: Rivers, Lakes, and Landscapes

Overview

  • Focus on the dynamic nature of rivers and lakes in shaping the landscape.

  • Reference study material: Planet Earth, Chapter 8.

Section 1: Fundamentals of Surface Water

  • Runoff and Groundwater Flow Statistics:

    • Annual runoff: 40,000 ext{ km}^3/ ext{yr}

    • Groundwater volume: 23,400,000 ext{ km}^3

    • Surface water volume: 190,000 ext{ km}^3

Section 2: Learning Outcomes

By the end of this section, you should be able to:

  1. Define key terms:

    • Overland flow: Water that flows across the surface of the ground when the soil is saturated.

    • Base flow: The portion of streamflow that comes from groundwater seepage into a river or stream.

    • Stream flow: Water that flows in a defined channel.

    • Discharge: The volume of water flowing through a river per unit time.

  2. Describe drainage basins and river systems.

  3. Identify processes of erosion and deposition in river systems.

  4. Evaluate statements about flooding risks related to rivers.

  5. Interpret the formation of open and closed lakes.

Section 3: Overland and Stream Flow

Overland Flow

  • Initial Process:

    • Rainfall or melted snow infiltrates soil initially.

    • After saturation, water flows downhill due to gravity, defined as overland flow.

Overland Flow Dynamics and Erosion

  • Fastest flow routes are high erosion sites.

  • Transition to Channel Flow: Overland flow can convert to channel flow as it concentrates in defined paths.

Stream Flow

  • Water flowing within established channels is termed stream flow.

  • Sources of Stream Flow:

    • Overland flow contributions.

    • Base flow from infiltrated groundwater.

Stream Flow Characteristics

  1. Gradient:

    • Measurement typically in m/km or ft/mile, sometimes expressed in degrees.

    • Example: N Saskatchewan River gradient is 0.3 ext{ m/km} (equivalent to 0.17°).

  2. Velocity:

    • Average velocity measured in m/s.

    • Variations in velocity based on location within the channel (slower near banks, faster in the middle).

    • Curved channels exhibit faster velocity on the outer curve.

Section 4: Discharge

  • Definition: Flux of water through a specific river point over time.

  • Calculation:

    • ext{Discharge} = ext{cross-sectional area} imes ext{velocity}

    • Units: volume per time (e.g. ext{m}^3/ ext{s} or ext{m}^3/ ext{day}).

    • Formula:

      • ext{Discharge} = ext{width} imes ext{depth} imes ext{velocity}.

Section 5: Hydrographs

  • Hydrographs: Record historical river discharge at specific points.

  • Factors influencing discharge include short-term events like storms or spring snowmelt.

  • Longitudinal Trends:

    • For North Saskatchewan River in Edmonton:

    • Total annual flow increased by 3.46 ext{%} from 1950–1979.

    • Total annual flow decreased by 5.29 ext{%} from 1980–2009, compared to 1912–1941.

Section 6: Drainage Basins and River Systems

River Roles in Landscape Formation

  1. Erosion of channels and valleys.

  2. Sediment transport from weathering.

  3. Sediment deposition leading to landform creation.

River Systems Structure

  • Streams organized into systems; small streams (tributaries) merge downstream.

  • Drainage Basin Definition:

    • Area drained by a major river and its tributaries, separated by watershed divides.

  • Watershed Clarification:

    • Often refers to the drainage basin in North America; in British contexts, indicates drainage divides.

Section 7: Continental-Scale Basins

  • Topographic Highs: Continental divides separate drainage basins flowing into different oceans.

Section 8: River Behaviour

  • Base Level: The lowest point a river can erode to, typically sea level or the level of a lake.

  • Classic river systems develop a graded profile, with gradient decreasing as they approach base level.

Flow Characteristics Downstream

  • As streams flow downstream, several characteristics change:

    1. Discharge increases.

    2. Width and depth both increase.

    3. Gradient decreases (e.g., steep mountain regions at 60 ext{ m/km}; N Saskatchewan at 0.3 ext{ m/km}).

    4. Velocity might increase slightly due to depth, but drag due to channel floor increases.

Section 9: Anatomy of a River

  • Rivers alter behaviour near base level:

    • Headwaters: deepening valleys via downcutting.

    • Middle sections: widening valleys through lateral erosion.

    • Lower sections: combining lateral erosion and deposition to form broad floodplains.

Section 10: Downstream Variation in Sediment Movement

  • Bed and Suspended Load: Deposited as gradients decrease, with suspended sediment needing calm water for deposition.

  • Dissolved Load: Often carried to the sea, depositing as non-clastic sediment like limestone or salt.

Section 11: Erosion Processes in River Valleys

  1. Downcutting: Lowering of stream bed occurs particularly in upper reaches.

  2. Mass Wasting: Includes landslides, rock falls, slides, and flows.

  3. Headward Erosion: Rapid downcutting leading to upstream erosion.

  4. Lateral Erosion: Dominates in lower river sections.

Section 12: Mass Wasting Types

Types of Mass Wasting:

  • Landslides: Material movement influenced by gravity.

  • Slides: Material movement along a single surface (translational and rotational types).

  • Flows: Mixed materials moving fluidly; includes:

    • Debris flows: Water-saturated material behavior.

    • Lahars: Debris flows triggered by volcanic activity.

  • Creep: Slow, gradual movement influenced by freeze-thaw cycles.

Section 13: Stream Piracy and Headward Erosion

  • Stream Capture: Occurs when one river diverts another, resulting in dry valleys.

  • Example: Gaspereau River headward erosion diverted Black River, drying Deep Hollow region.

Section 14: Depositional Landforms in River Systems

  • Various landforms formed via sediment deposition:

    • Alluvial fans

    • Braided rivers

    • Meandering channels

    • Floodplains

    • Deltas and estuaries

Section 15: Alluvial Fans

  • Characteristics: Fan-shaped sediment accumulations at valley openings. Typically in drier climates with sudden loss of stream power.

Section 16: River Morphology

  • Focus on braided and meandering channels:

    • Braided Rivers: Characterized by multiple, interwoven channels, high sediment load.

    • Meandering Channels: Form sinuous loops, moderate to low sediment supply, and cohesive banks.

Section 17: Meander Migration and Oxbow Lakes

  • Meandering Process: Erosion occurs outside the curve, with deposition inside, leading to migration of meanders into broader patterns.

  • Oxbow Lakes: Result from cut-off meanders, forming through deposition events.

Section 18: Flooding Mechanisms

  • Flood Events: Occur when discharge exceeds channel capacity, transporting sediments.

  • Floodplain Importance: Fertile land attracting settlements, despite the hazard risk.

Historical Contexts of Flooding

  • Specific events discussed for places like Calgary and Edmonton focusing on the 1915 events and sediment deposition.

Section 19: Risk Assessment via Hydrographs

  • Hydrographs are utilized to assess long-term flood risk based on recorded discharge.

Section 20: Recurrence Intervals

  • Recurrence Interval Definition: Average time between floods of the same magnitude.

    • E.g., a “10-year flood” occurs with a 10 ext{%} chance each year.

Section 21: Deltas and Estuaries

  • Form when rivers deposit sediment into standing water bodies, producing diverse ecological habitats.

  • Estuaries represent mixed river and seawater areas.

Section 22: Base Level Changes

  • Influences of sea-level changes, tectonic activities, blockages, and land uplift on river dynamics.

Section 23: Landforms and Terraces

  • Formation of terraces as rivers alternate between lateral erosion and downcutting.

Section 24: Incised Meanders

  • Rapid base level decreases can produce incised meanders, characterized by steep banks and distinct shapes.

Section 25: Superimposed Drainage Systems

  • Existing river channels may persist through eroded landscapes, exemplified by the Susquehanna River in Pennsylvania.

Section 26: Lakes

  • Lake Definition: Standing water bodies created by depression or physical obstruction in flow.

  • Key obstacles: Glacial activity, volcanic formations, and geomorphological conditions.

Lake Dynamics and Lifespan

  • Lakes can serve as local base levels but are typically short-lived geologically due to erosion, sedimentation, or evaporation processes.

Open and Closed Lakes

  • Open Lakes: Have inflow and outflow, maintaining lower saline levels.

  • Closed Lakes: Lack outlets leading to salinity due to evaporation.

Conclusion

  • Understanding river and lake systems is crucial for assessing ecological balance, sediment dynamics, and flood risks in landscapes.