🧭Physical Geography Unit 11 – Hydrosphere: Oceans, Coasts, and Groundwater

Key Concepts and Definitions

• Hydrosphere encompasses all water on Earth's surface, including oceans, lakes, rivers, and groundwater
• Salinity measures the amount of dissolved salts in water, with oceans having an average salinity of 35 parts per thousand (ppt)
• Thermocline a layer in the ocean where temperature changes rapidly with depth, separating the mixed layer from deep ocean waters
• Coriolis effect deflects moving objects, including ocean currents and winds, to the right in the Northern Hemisphere and left in the Southern Hemisphere due to Earth's rotation
• Longshore drift the movement of sediment along a coastline caused by waves approaching the shore at an angle
• Aquifers porous and permeable rock layers that store and transmit groundwater
  • Confined aquifers are bounded by impermeable layers above and below
  • Unconfined aquifers have no upper confining layer and are open to the atmosphere
• Saltwater intrusion occurs when saltwater moves into freshwater aquifers, often due to excessive groundwater pumping in coastal areas

Ocean Composition and Structure

• Oceans cover approximately 71% of Earth's surface and contain 97% of its water
• Seawater composition includes dissolved salts, gases, and organic matter
  • Major ions in seawater sodium (Na+), chloride (Cl-), magnesium (Mg2+), and sulfate (SO42-)
• Ocean layers from top to bottom mixed layer, thermocline, and deep ocean
  • Mixed layer is well-mixed by wind and waves, with relatively uniform temperature
  • Deep ocean is cold, dense, and has a relatively uniform temperature around 2-3°C
• Ocean basins are divided into major features continental shelves, continental slopes, abyssal plains, and ocean trenches
• Seafloor spreading occurs at mid-ocean ridges, where new oceanic crust is formed as magma rises and solidifies
• Subduction zones are where oceanic crust is recycled back into the mantle, often forming deep ocean trenches (Mariana Trench)

Global Ocean Circulation

• Ocean currents are driven by wind, density differences, and Earth's rotation (Coriolis effect)
• Surface currents are primarily wind-driven and form large circular patterns called gyres
  • Major surface currents include the Gulf Stream, Kuroshio Current, and Antarctic Circumpolar Current
• Thermohaline circulation (THC) is a deep-ocean circulation driven by density differences caused by temperature and salinity variations
  • THC is also known as the "global conveyor belt" and plays a crucial role in redistributing heat and nutrients globally
• Upwelling occurs when deep, nutrient-rich waters are brought to the surface, often along coastlines (Peru, California)
• El Niño-Southern Oscillation (ENSO) is a periodic fluctuation in ocean temperatures and atmospheric pressure in the Pacific Ocean that affects global weather patterns
  • El Niño warm phase, characterized by warmer surface waters in the eastern Pacific
  • La Niña cool phase, characterized by cooler surface waters in the eastern Pacific

Coastal Processes and Landforms

• Waves are generated by wind blowing over the ocean surface and transfer energy to the shoreline
  • Wave characteristics include height, wavelength, and period
• Tides are the regular rise and fall of sea level caused by the gravitational pull of the Moon and Sun
  • Tidal range the difference between high and low tide, varies depending on location and lunar phase
• Coastal erosion is the removal of sediment from the shoreline by waves, currents, and tides
  • Erosional features include cliffs, wave-cut platforms, and sea arches
• Coastal deposition occurs when sediment is transported and deposited along the shoreline
  • Depositional features include beaches, spits, and barrier islands
• Deltas form where rivers deposit sediment as they enter the ocean (Nile Delta, Mississippi Delta)
• Estuaries are partially enclosed coastal bodies of water where freshwater from rivers mixes with saltwater from the ocean (Chesapeake Bay)

Groundwater Systems and Aquifers

• Groundwater is water stored in the pores and fractures of soil and rock beneath the Earth's surface
• Porosity the percentage of void space in a rock or sediment that can store water
• Permeability the ability of a rock or sediment to transmit water through its pores or fractures
• Water table the upper surface of the saturated zone in an unconfined aquifer
• Recharge the process by which water is added to an aquifer, typically from precipitation or surface water infiltration
• Discharge the process by which water leaves an aquifer, such as through springs, seeps, or pumping wells
• Karst landscapes form in areas with soluble bedrock (limestone, dolomite) and are characterized by sinkholes, caves, and underground drainage systems

Human Impact on Hydrosphere

• Water pollution occurs when harmful substances (chemicals, pathogens, nutrients) enter water bodies
  • Point sources of pollution originate from a single, identifiable location (factories, sewage treatment plants)
  • Non-point sources of pollution come from diffuse sources (agricultural runoff, urban stormwater)
• Eutrophication the excessive growth of algae and aquatic plants due to nutrient enrichment, leading to oxygen depletion and ecosystem degradation
• Groundwater depletion occurs when the rate of groundwater extraction exceeds the rate of recharge
  • Subsidence the gradual settling or sinking of the ground surface due to groundwater depletion
• Dams and reservoirs alter river flow, sediment transport, and aquatic habitats
  • Benefits of dams include water storage, flood control, and hydroelectric power generation
  • Negative impacts of dams include habitat fragmentation, changes in water temperature and quality, and sediment trapping
• Coastal development can lead to increased erosion, habitat loss, and water quality degradation
  • Hardening shorelines with seawalls and jetties can disrupt natural coastal processes and ecosystems

Climate Change and the Hydrosphere

• Rising global temperatures lead to thermal expansion of seawater and melting of land-based ice, causing sea-level rise
  • Coastal communities and low-lying islands are particularly vulnerable to sea-level rise and increased flooding
• Changes in precipitation patterns can lead to more frequent and intense droughts and floods
  • Droughts can strain water resources, impact agriculture, and increase wildfire risk
  • Floods can cause damage to infrastructure, displace communities, and degrade water quality
• Ocean acidification occurs as the ocean absorbs excess atmospheric carbon dioxide (CO2), lowering the pH of seawater
  • Lower pH can negatively impact marine organisms that build calcium carbonate shells and skeletons (corals, mollusks)
• Warming ocean temperatures can alter ocean circulation patterns, affect marine species distributions, and increase the frequency and intensity of tropical cyclones
• Melting permafrost in Arctic regions can release stored greenhouse gases (methane, CO2) and alter hydrological processes

Case Studies and Real-World Applications

• Ogallala Aquifer a vast groundwater system underlying parts of eight U.S. states, facing depletion due to intensive irrigation
  • Conservation efforts include more efficient irrigation techniques and switching to less water-intensive crops
• Chesapeake Bay the largest estuary in the U.S., experiencing water quality issues due to nutrient pollution from agriculture and urban development
  • Restoration efforts involve reducing nutrient inputs, restoring wetlands and oyster reefs, and implementing best management practices
• Maldives a low-lying island nation in the Indian Ocean, facing existential threat from sea-level rise
  • Adaptation strategies include building seawalls, elevating infrastructure, and exploring relocation options
• Great Pacific Garbage Patch a large accumulation of plastic debris in the North Pacific Ocean, highlighting the global issue of marine plastic pollution
  • Mitigation efforts include reducing plastic consumption, improving waste management, and developing ocean cleanup technologies
• Aral Sea a once-large saltwater lake in Central Asia that has shrunk dramatically due to water diversion for irrigation
  • Consequences include loss of fisheries, increased salinity, and public health issues from exposed lakebed sediments