Hydrosphere and River Dynamics: Water Cycle, Stream Features, and Erosion

Hydrosphere

The portion of Earth's water system including oceans, rivers, lakes, groundwater, and ice that interacts with the atmosphere, biosphere, and lithosphere

Four steps of running water in the hydrologic cycle

1) Evaporation 2) Precipitation 3) Melting 4) Runoff

Advection

Horizontal transport of water vapor through the atmosphere from one region to another

Throughflow

Water that moves laterally through the soil layer downslope, between the surface and the water table

Percentage of Earth's water in the oceans

96.5% of total global water is salt water in the oceans

Percentage of Earth's water that is freshwater

Only 2.5% of total global water is freshwater

Where most freshwater is stored

68.6% of freshwater is locked in glaciers and ice caps

Percentage of freshwater that is groundwater

30.1% of Earth's freshwater is groundwater

Percentage of surface freshwater in rivers

Only 0.46% of surface water and other freshwater is in rivers

Percentage of surface freshwater in lakes

20.1% of surface water and other freshwater is in lakes

Permanent stream

A stream that flows year-round, sitting at or below the water table; found in tropical or temperate climates with sufficient rainfall and low evaporation

Ephemeral stream

A stream that dries up part of the year, sitting above the water table; found in dry climates with low rainfall and high evaporation

Stream discharge

The volume of water flowing through a channel per second (m³/s)

Discharge formula

D = W × d × V (Discharge = Width × Depth × Velocity)

Amazon River discharge

200,000 m³/s — highest discharge of any river on Earth

Congo and Mississippi River discharge

Both approximately 40,000 m³/s

Stream gradient

The slope or steepness of a stream channel; highest near the source and lowest near the mouth

Braided stream

A stream near high gradients that divides into numerous interweaving strands separated by sandbars

Meandering stream

A stream near base level with low gradient, broad channels, high discharge, low velocity, and sinuously curving channels

Source of a stream

The starting point of a stream at high elevation where gradient is steepest and erosion dominates

Tributary

A smaller stream that flows into a larger trunk stream; associated with strong flow and erosion

Trunk stream

The main river channel into which tributaries flow; associated with meandering

Delta

A depositional feature at the mouth of a river where sediment is dropped as the river meets standing water

Longitudinal profile

A curve showing a stream's elevation from source to mouth — steep near source, gentle near base level

Base level

The lowest point to which a stream can erode

Ultimate base level

Sea level — the lowest point any stream can theoretically erode to

Local (temporary) base level

A temporary obstacle like a lake, dam, or resistant rock layer that acts as a local base level for a stream

Effect of changing base level

If base level drops, the stream erodes; if base level rises, the stream deposits sediment

Why waterfalls form

Waterfalls form at a local base level where resistant rock creates a vertical drop; they migrate upstream over time

Scouring

Stream erosion where the force of flowing water breaks apart wall rock

Abrasion (stream)

The grinding/sandpaper-like action of sediment-laden water wearing down rock surfaces

Dissolution (stream)

Chemical weathering where water dissolves water-soluble minerals from rock

Breaking and lifting

Stream erosion where flow breaks apart bottom rock and suspends fragments in the water

Pothole

A rounded hole in stream bedrock formed by abrasion from swirling cobbles near turbulent flows

When stream erosion is greatest

During floods, because higher velocity greatly increases erosive power

How velocity affects sediment deposition

As velocity decreases, the stream deposits sediment — largest clasts first, finest sediments last

Which sediments travel furthest downstream

Silts and clays (smallest grains) — they remain suspended longest

Floodplain

Broad flat regions on either side of a river channel covered by water and sediment during floods, producing fertile soils

Natural levees

Raised ridges of coarse-grained sediment deposited along river banks during repeated floods

Why floodplain soils are fertile

Floods deposit nutrient-rich fine sediments across the floodplain over time

Cutbank

The outer, eroding bank of a meander where water moves fastest

Point bar

The inner, depositional side of a meander where water moves slowest and sediment accumulates

Oxbow lake

A crescent-shaped lake formed when a meander is cut off from the main river channel

How an oxbow lake forms

Erosion on cutbanks and deposition on point bars make meanders more extreme until the neck is cut through, isolating the loop

Narrow valley

A valley dominated by erosion where the stream has more energy than sediment load — found at high gradients

Wide valley

A valley where the stream has more sediment than it can carry, resulting in net deposition and a broad valley floor

Intermediate valley

A valley in balance between erosion and deposition

Drainage basin (watershed)

The region drained by a stream and all its tributaries — all water in the basin flows to the same outlet

Drainage divide

A ridge or high ground that separates two adjacent watersheds

Continental divide

A continental-scale drainage divide separating watersheds draining into different oceans

Dendritic drainage

A tree-like branching stream pattern that develops on uniform rock — the most common drainage pattern

Why the Amazon reversed flow direction

During Pangean times rivers flowed west; after the Andes uplifted, drainage reversed to flow east into the Atlantic

Mississippi River daily sediment load

1.4 million tons of sediment per day — equal to the weight of 500,000 cars

Subsidence in deltas

Deltas sink over time as sediment compacts, making them vulnerable to flooding and sea level rise

Man-made levees

Engineered embankments along rivers to prevent overflow; they can cause soil instability, downstream flooding, and are expensive

Problems with man-made levees

Prevent natural floodplain recharge, cause soil erosion and instability, transmit flood problems downstream, and are costly

Hurricane Katrina economic impact

Approximately $160 billion in damage — the costliest U.S. hurricane on record

Live with water philosophy

Adapting to water rather than fighting it (e.g., Netherlands farming below sea level using polders and drainage systems)

Number of ocean basins

5 ocean basins (Pacific, Atlantic, Indian, Arctic, Southern) — all part of one world ocean

Average ocean depth

3,688 meters

How ocean basins differ from continents

Oceans lack freeze-thaw fragmentation, have slow chemical weathering, and are dominated by volcanism and sedimentation

Age range of ocean crust

0-180 Ma — much younger than continental crust (0-4,000 Ma)

Where tectonic activity occurs in ocean basins

At mid-ocean ridges (spreading) and subduction zones

Shoreline

The contact zone between land and sea

Coastline

The seaward edge of the coast, landward of the shoreline

Percentage of world population within 100 km of coast

About 40%

Foreshore

The zone between low-tide and high-tide shorelines — the beach face exposed and covered by tides

Backshore

The zone above the high-tide shoreline, affected only by storm waves

Berm

A nearly horizontal accumulation of sediment at the top of the beach face marking the usual high-water limit

Factors shaping shorelines

Sand input/output, tectonic uplift/subsidence, rock type, sea level changes, tidal height, and wave energy

Sand budget

The balance between sand inputs (cliff erosion, longshore drift, rivers) and outputs (dunes, offshore transport) on a beach

Wave erosion causes

Wave impact and pressure break down rock; abrasion from rock-fragment-laden water grinds surfaces

Longshore current

A current running parallel to the shoreline generated when waves strike the coast at an angle

Longshore drift

The net movement of sediment along the shoreline driven by the longshore current

Spit

A finger of sand with one end connected to land and the other extending into open water, formed by longshore drift

Sandbar / barrier island

An offshore or shore-connected accumulation of sand built by wave action; can form a long linear barrier island like Miami Beach

Sea arch

An erosional coastal feature formed when waves erode through a headland leaving a rock arch

Wave-cut platform

An erosional surface of bare rock at the base of a retreating sea cliff

Groin

A hard structure built perpendicular to the beach to trap sand moving via longshore drift

Breakwater

A barrier built offshore and parallel to the coast to protect boats from wave energy

Seawall

An armored barrier along the coast to absorb or reflect breaking wave energy

Rip rap

Large rocks piled along a shore to dissipate wave energy and protect against erosion

Beach nourishment

Adding sand from an outside source to restore an eroding beach — a soft stabilization method

Three responses to shoreline erosion

1) Hard stabilization (groins, seawalls), 2) Relocation (moving buildings back), 3) Beach nourishment (adding sand)

Why hard stabilization often fails long-term

It interrupts the natural sediment budget and causes erosion elsewhere without addressing underlying dynamic processes

Atlantic/Gulf Coast vs. Pacific Coast erosion

Atlantic/Gulf: wide beaches, barrier island flooding risk; Pacific: narrow beaches backed by steep cliffs, sediment trapped by dams