🌋Physical Geology Unit 12 – Surface Water and Fluvial Processes

Introduction to Surface Water

• Surface water refers to water found on the Earth's surface, including rivers, lakes, streams, and oceans
• Accounts for a small percentage of the Earth's total water supply, with most water stored in oceans, ice caps, and groundwater
• Plays a crucial role in shaping the Earth's landscape through erosion, transport, and deposition of sediments
• Influences climate by absorbing and releasing heat, as well as through evaporation and precipitation processes
• Supports a wide variety of ecosystems and is essential for human activities such as agriculture, industry, and recreation
• Can be impacted by human activities, such as pollution, dam construction, and land use changes, leading to altered water quality and quantity

Hydrologic Cycle and Water Distribution

• The hydrologic cycle, also known as the water cycle, describes the continuous movement of water on, above, and below the Earth's surface
• Driven by solar energy and gravity, the cycle involves processes such as evaporation, transpiration, condensation, precipitation, infiltration, and runoff
• Evaporation occurs when water changes from a liquid to a gas, primarily from oceans, lakes, and rivers (approximately 86% from oceans)
• Transpiration is the process by which water is released into the atmosphere through plant leaves
• Condensation happens when water vapor cools and forms liquid water droplets, leading to the formation of clouds and fog
• Precipitation occurs when water droplets become heavy enough to fall back to the Earth's surface as rain, snow, sleet, or hail
• Infiltration is the process by which water enters the soil and moves downward through pores and cracks (approximately 50% of precipitation infiltrates)
• Runoff is the portion of precipitation that flows over the land surface towards streams, rivers, and lakes (approximately 35% of precipitation becomes runoff)
  • Surface runoff occurs when the rate of precipitation exceeds the rate of infiltration, leading to overland flow
  • Subsurface runoff, or interflow, occurs when water moves laterally through the soil and eventually reaches a stream or river

River Systems and Drainage Patterns

• A river system consists of a main river channel and its tributaries, which are smaller streams that feed into the main channel
• The area of land that contributes water to a river system is called a drainage basin or watershed, separated by topographic divides
• The shape and characteristics of a drainage basin are influenced by factors such as climate, geology, and topography
• Drainage patterns refer to the spatial arrangement of streams and rivers within a drainage basin
• Common drainage patterns include:
  • Dendritic: Resembles the branching pattern of a tree, develops in areas with uniform rock resistance (e.g., Mississippi River basin)
  • Trellis: Characterized by parallel main streams with short, perpendicular tributaries, forms in areas with alternating resistant and weak rock layers (e.g., Appalachian Mountains)
  • Radial: Streams flow outward from a central high point, such as a volcano or dome (e.g., Mount Kilimanjaro)
  • Rectangular: Streams follow a grid-like pattern, influenced by joints and faults in the underlying bedrock (e.g., Colorado Plateau)
• The size and shape of a drainage basin can influence the timing and magnitude of stream flow and flooding events

Stream Flow and Discharge

• Stream flow refers to the movement of water through a channel, influenced by factors such as gradient, channel shape, and roughness
• Discharge is the volume of water flowing past a specific point in a stream per unit time, typically measured in cubic meters per second (m³/s) or cubic feet per second (ft³/s)
• Discharge is calculated using the equation: $Q = A × V$, where $Q$ is discharge, $A$ is the cross-sectional area of the stream, and $V$ is the average velocity of the water
• Factors affecting stream velocity include:
  • Gradient: Steeper gradients result in faster water flow
  • Channel roughness: Rougher channels (e.g., those with boulders or vegetation) slow down water flow
  • Channel shape: Narrow, deep channels generally have faster flow than wide, shallow channels
• Stream flow can be laminar (smooth, parallel flow lines) or turbulent (chaotic, mixing flow), depending on factors such as velocity, depth, and channel roughness
• Discharge varies over time, with high flows during periods of heavy precipitation or snowmelt and low flows during dry periods
• The long-term pattern of stream flow in a given location is called the flow regime, which can be influenced by climate, geology, and land use changes

Erosion and Sediment Transport

• Erosion is the process by which water, wind, or ice detaches and removes rock particles or soil from the Earth's surface
• In fluvial systems, erosion occurs through processes such as abrasion, hydraulic action, and solution
  • Abrasion is the grinding of rock particles against each other or the stream bed, leading to the reduction of particle size
  • Hydraulic action involves the force of moving water dislodging and entraining particles
  • Solution is the chemical weathering of rocks by dissolved acids or other substances in the water
• Sediment transport refers to the movement of eroded particles by flowing water, which can occur as bedload, suspended load, or dissolved load
  • Bedload consists of larger particles that roll, slide, or bounce along the stream bed
  • Suspended load includes smaller particles that are carried within the water column by turbulence
  • Dissolved load refers to ions and molecules that are dissolved in the water
• The capacity of a stream to transport sediment depends on factors such as water velocity, depth, and channel gradient
• Hjulström curve illustrates the relationship between water velocity and particle size in terms of erosion, transport, and deposition
• Sediment is deposited when the stream's capacity to transport particles decreases, such as when velocity slows down or the channel widens

Channel Morphology and Floodplains

• Channel morphology refers to the shape and characteristics of a stream channel, which can vary along its length and over time
• Factors influencing channel morphology include:
  • Discharge: Higher discharges generally lead to wider, deeper channels
  • Sediment load: The amount and size of sediment transported by the stream affects channel shape and stability
  • Gradient: Steeper gradients often result in narrower, deeper channels with faster flow
  • Bank materials: The composition and erodibility of the channel banks influence channel width and stability
• Stream channels can be classified as straight, meandering, or braided, depending on their sinuosity and number of channels
  • Straight channels have a single, relatively straight course and are rare in nature
  • Meandering channels have a single, sinuous course with alternating pools and riffles (e.g., lower Mississippi River)
  • Braided channels consist of multiple, intertwining channels separated by sediment bars (e.g., Brahmaputra River)
• Floodplains are the relatively flat areas adjacent to a stream channel that are subject to periodic inundation during high flows
• Floodplains are formed by the deposition of sediment during overbank flooding events and can contain features such as natural levees, backswamps, and oxbow lakes
• The frequency and magnitude of flooding on a floodplain are influenced by factors such as climate, land use, and channel morphology
• Human activities, such as urbanization and channelization, can alter the natural functioning of floodplains and increase the risk of flood damage

Landforms Created by Fluvial Processes

• Fluvial processes, such as erosion, transport, and deposition, create a variety of landforms in river systems
• Erosional landforms include:
  • V-shaped valleys: Formed by the downcutting of streams in areas with resistant bedrock (e.g., Grand Canyon)
  • Waterfalls: Created when a stream flows over a resistant rock layer underlain by a less resistant layer, leading to undercutting and collapse (e.g., Niagara Falls)
  • Potholes: Circular depressions formed by the grinding action of sediment in swirling eddies
• Depositional landforms include:
  • Alluvial fans: Fan-shaped deposits that form where a stream emerges from a confined valley onto a flatter plain (e.g., Death Valley)
  • Deltas: Triangular-shaped deposits that form where a river enters a standing body of water, such as a lake or ocean (e.g., Mississippi River Delta)
  • Point bars: Crescent-shaped deposits that form on the inside of river bends due to slower water velocity and sediment deposition
  • Floodplains: Relatively flat areas adjacent to a stream channel that are built up by the deposition of sediment during overbank flooding events
• The formation and evolution of these landforms are influenced by factors such as climate, geology, and base level changes
• Understanding the processes and landforms associated with fluvial systems is important for hazard assessment, resource management, and infrastructure planning

Human Impact and Water Management

• Human activities have a significant impact on surface water systems, altering the quantity, quality, and distribution of water resources
• Land use changes, such as deforestation, urbanization, and agriculture, can affect the hydrologic cycle by altering infiltration rates, runoff patterns, and evapotranspiration
• Urbanization increases the area of impervious surfaces, such as roads and buildings, leading to higher runoff rates and increased flood risk
• Agricultural practices, such as irrigation and the application of fertilizers and pesticides, can impact water quality and alter the natural flow regime of streams and rivers
• Dam construction and river regulation can alter the natural flow regime, sediment transport, and ecosystem dynamics of rivers
  • Dams can provide benefits such as flood control, hydroelectric power generation, and water storage for irrigation and municipal use
  • However, dams can also have negative impacts, such as the disruption of fish migration, changes in downstream sediment and nutrient transport, and the alteration of floodplain ecosystems
• Water pollution, from sources such as industrial discharges, sewage, and agricultural runoff, can degrade water quality and harm aquatic ecosystems
• Water management strategies aim to balance the competing demands for water resources while minimizing negative impacts on the environment
• Integrated water resource management (IWRM) is an approach that considers the interdependence of water, land, and related resources, and seeks to optimize economic and social welfare without compromising the sustainability of ecosystems
• Best management practices (BMPs) can be implemented to mitigate the impacts of human activities on water resources, such as:
  • Riparian buffer zones: Vegetated areas along stream banks that filter pollutants and stabilize banks
  • Stormwater management: Techniques to reduce runoff and improve water quality, such as permeable pavement and green roofs
  • Agricultural BMPs: Practices to reduce soil erosion and nutrient runoff, such as conservation tillage and precision fertilizer application
• Effective water management requires collaboration among stakeholders, including government agencies, industry, agriculture, and the public, to develop and implement sustainable solutions