ESCI 121 Lecture 16: The Geology of Running Water
Hydrology and the Hydrologic Cycle
Hydrology Defined: The scientific study of the movement, distribution, and quality of water throughout the Earth. This field addresses the hydrologic cycle and management of water resources.
Hydrogeology Defined: A specialized sub-field of hydrology that focuses on the distribution and movement of groundwater within the soil and rocks of the Earth's crust.
The Hydrologic Cycle: Represents a comprehensive summary of the continuous circulation of the Earth’s water supply. - Earth's water resides in various reservoirs. - Water transitions between these reservoirs along pathways driven by diverse physical processes. - Total Reservoir Volume: Approximately ().
Key Hydrologic Processes and Pathways: - Evaporation: The phase change of water from liquid to vapor, resulting in the removal of water from surfaces (land and water) into the atmosphere. - Transpiration: The process in which plants release water vapor into the atmosphere. - Evapotranspiration: The combined sum of evaporation and transpiration. - Precipitation: Moisture falling from the atmosphere, including rain, snow, and sleet. - Surface Runoff: Water that flows over the ground surface to eventually join a surface water body. - Infiltration: The process of water percolating into the ground from the surface. - Groundwater: Water that is moving and stored within the pore spaces of underground rocks. - Ocean Currents: Movement of water through surface and thermohaline (deep-water) circulation.
USGS Water Cycle Diagram: A global reference tool provided by the USGS, available in 60 different languages to support worldwide water science education.
Stream Formation and Running Water
Precipitation and Infiltration: The amount of precipitated water that infiltrates the subsurface is determined by the Infiltration Capacity. - Factors controlling infiltration capacity: - Total amount, intensity, and duration of rainfall. - The prior wetted condition of the soil. - Soil texture (porosity and permeability). - The slope of the land (steeper slopes favor runoff over infiltration). - The nature and density of vegetative cover.
Development of Channels: Surface runoff initiates as sheetflow (overland flow not in channels). - Sheetflow develops into tiny channels called rills. - Rills grow into larger gullies. - Gullies eventually evolve into permanent channels.
Stream Terminology: - Streams: Surface waters flowing within defined channels, except during flood events when they go overbank onto the floodplain. - Rivers: A term typically reserved for larger streams. - Brooks and Creeks: Common names for smaller streams. - Fluvial: The primary adjective used researchers to describe rivers and streams. - Tributaries: Smaller streams that feed into a larger primary stream.
River Systems: A network of connecting channels. - Sources: Melting snow/ice, precipitation, groundwater, and other rivers. - Function: They are the dominant agent of erosion on Earth and transport weathered rock debris. - Structure: Water flows from the headwaters (source), grows as a trunk via tributaries, and empties at the mouth.
Drainage Basins and Patterns
Drainage Basin (Watershed/Catchment): The total land area that contributes water to a specific stream. - Bounded by a geographic high point called a divide. - Structure is Hierarchical: Made of many small sub-basins nested within fewer, larger basins.
Major Examples: - The Mississippi Drainage Basin is the largest in the United States. - Great Basin (North America): A unique system where rivers do not drain into a sea or ocean but instead empty into internal lakes and playas.
Drainage Patterns: Surface map-view patterns controlled by underlying geologic structures. - Dendritic: Tree-like, branching patterns typically found on uniform material. - Rectangular: Characterized by right-angle bends, often following joint or fault systems. - Trellis: Parallel main streams with short tributaries entering at right angles, found in folded terrains. - Radial: Streams radiating outward from a central high point, such as a volcano or dome. - Deranged: Highly irregular patterns lacking clear geometry, often found in recently glaciated areas with poor drainage.
Stream Dynamics: Interaction, Discharge, and Measurement
Interaction with Groundwater: - Gaining Streams: Common in humid climates; the water table intersects the stream channel, and groundwater reemerges as surface flow. - Losing Streams: Common in arid climates; water infiltrates from the stream channel into the ground because the water table is below the channel.
Control Factors of Stream Behavior: Includes channel cross-sectional area, shape, discharge, gradient (slope), average velocity, and sediment load.
Discharge (): - Definition: The volume of water moving past a given point in a specific amount of time. - Formula: (Area Mean Velocity). - Units: Typically reported in cubic feet per second ( or cfs) or cubic meters per second (). - Discharge variations occur over time (daily, seasonal, annual) and space (along the river’s course).
Measuring Stream Flow: - Stream Gaging Station: A facility that measures the stage (river level). - To calculate discharge, depth is measured at regular width () intervals to find area, and flow meters measure velocity. - Rating Curve: A graphical tool that converts recorded water level (stage) into discharge values based on previously calculated measurements.
Floods and Flood Frequency
Hydrograph: A record of stream discharge over time. - Lag Time: The duration between the start of a precipitation event and the occurrence of peak discharge (flood crest). - Large rivers may have lag times of days or weeks. - Small streams may have lag times of a few hours; very short lag times indicate a flash flood.
Flood Characteristics: Floods occur when runoff exceeds the land's absorption capacity and the stream overflows into the floodplain. - Human Impact: Floods are the most frequent and lethal of all natural disasters.
Historical Flood Events: - 1993 Upper Mississippi River Flood: The worst US flood disaster, causing property damage exceeding . - 2019 Midwest US Floods: Caused multi-billion dollar damages (e.g., Hamburg, Iowa).
Flood Frequency Analysis: - Recurrence Interval (): Relates the magnitude of floods to their frequency. - Probability (): Calculated as . - Example: A 10-year flood has a probability of () of occurring in any given year. This does not mean it happens exactly once every 10 years. - These estimates are vital for flood insurance and zoning activities.
Sediment Transport and Deposition
Sediment Load: The total volume of sediment carried by a stream. - Dissolved Load: Ions in solution (the "invisible load"). - Suspended Load: Fine particles kept in the water column by turbulence. - Bed Load: Coarse particles moving by rolling, sliding, or bouncing (saltation).
Sediment Sources: 1. Water Erosion: Weathered particles picked up by surface runoff. 2. Mass Movement: Materials moving downslope into the channel via gravity. 3. Bank Erosion: The stream itself eroding the sides of its own channel.
Transport Metrics: - Competence: The maximum clast size a stream can carry. - Capacity: The total quantity of sediment a stream carries. - Both change relative to discharge; high discharge allows for the movement of large cobbles and boulders.
Deposition Factors: Sediment is deposited when energy decreases. This occurs when water depth decreases (friction increases), slope decreases, or velocity decreases.
Stream Profiles and Longitudinal Trends
Longitudinal Profile: A smooth curve showing the cross-sectional view of a stream from head to mouth. - Stream Gradient: The vertical change of a channel over a horizontal distance (). Gradient typically decreases downstream.
Downstream Trends: - Factors that Increase Downstream: Discharge, channel size, and velocity. - Factors that Decrease Downstream: Gradient (slope), channel roughness, and sediment grain size.
Headwaters (Source): Small streams in high-elevation areas. Typically feature straight channels, steep gradients, high competence, and coarse sediments.
Mouth: The end of the river where it enters a larger body of water. Features low gradients, high discharge, low competence, meandering channels, and fine sediments.
Base Level: - Definition: The lowest point to which a stream can erode. This is the elevation where potential energy is zero. - Ultimate Base Level: The ocean. - Dynamics: Raising the base level leads to deposition; lowering it leads to erosion and channel incision.
Graded Stream: A stream in equilibrium where velocity, volume, gradient, and load are balanced. An ungraded stream will adjust its variables to become graded.
Fluvial Erosional Features and Valley Evolution
Drainage Network Evolution: Channels deepen via downcutting and lengthen via headward erosion.
Headward Erosion Case Study: Niagara Falls has been eroding its gorge upstream for approximately since the last ice age, seeking to re-establish equilibrium after a drop in base level.
Valleys vs. Canyons: - Valley: Gently sloping sidewalls and wider bottom; mountain valleys are often V-shaped. - Canyon: Steep sidewalls and narrow bottom. - Gorge: An exceptionally narrow, steep-walled canyon. - Rock resistance affects profile: Strong rocks form cliffs; weak rocks form gentle slopes.
Incised Meanders: Meander loops found in steep, narrow valleys. They form through rapid downcutting into hard bedrock caused by a drop in base level or regional uplift (e.g., Colorado River).
Stream Terraces: Remnants of former floodplains that have been abandoned. They indicate that a river has adjusted to a drop in base level by downcutting to a new elevation.
Channel Patterns and Landforms
Straight Channels: Minimal curving, often found in high-gradient headwaters.
Braided Streams: Multichannel systems with many interlocking loops. - Environment: Near mountains or glaciers with steep slopes. - Sediments: Coarse-grained (sand and gravel). - Flow: "Flashy" discharge (e.g., New Zealand).
Meandering Streams: Single channels with wide, sinuous loops. - Environment: Low slope areas near the mouth of the river. - Sediments: Fine-grained (sand and mud). - Flow: Steady discharge (e.g., Canada).
Meander Dynamics: - Cut Bank: The outer bank of a meander where high velocity causes erosion. - Point Bar: The inner bank where lower velocity leads to deposition. - Oxbow Lake: A U-shaped body of water formed when a meander loop grows so large it is cut off from the main channel.
Alluvial Landforms: - Floodplains: Flat land adjacent to a channel built by lateral channel shifting and overbank deposition. - Natural Levees: Elevated ridges beside the channel formed when sediment is deposited during flood overflows. - Alluvial Fan: A fan-shaped deposit at the base of a mountain or across a fault where a river leaves a confined valley and velocity drops (e.g., Canterbury, NZ). - Delta: An accumulation of sediment where a stream enters a standing body of water. Named by Herodotus (approx. ) for the Greek letter shape.
The Mississippi Delta: Exhibits a "Bird-foot" pattern with active distributaries. Over the last , it has undergone lobe switching as the river seeks shorter paths to the sea.
Estuaries: Formed in place of deltas when sea-level rise outpaces the sediment delivery flux of the river (e.g., Bristol Estuary, UK).