APES Unit 4 Water Systems: Drainage Basins and Climate Oscillations

Watershed (Drainage Basin/Catchment)

An area of land where all precipitation drains downhill to the same outlet (e.g., creek, river, lake, estuary, or ocean), defined by topography.

Drainage Divide

A ridge or higher-elevation boundary that separates two watersheds; water falling on opposite sides flows to different outlets.

Main Channel

The primary stream/river channel that collects water (and materials) from the entire surrounding watershed.

Nested Watersheds

The idea that small watersheds drain into larger ones (creek → river → ocean basin), so upstream actions can affect many downstream areas.

Infiltration

The process of water soaking into the soil; generally higher in porous, vegetated, uncompacted soils and lower in clay-rich or paved areas.

Percolation

Downward movement of infiltrated water through soil/rock toward groundwater storage.

Groundwater Recharge

The addition of water to groundwater supplies when water infiltrates and percolates downward; depends on infiltration capacity and soil saturation, not just rainfall amount.

Aquifer

An underground layer of permeable rock or sediment that stores and transmits groundwater.

Baseflow

The steady contribution of groundwater to streamflow between storms, helping maintain streamflow during dry periods.

Surface Runoff

Water that flows over the land surface into streams when infiltration is limited (e.g., steep slopes, saturated soils, clay soils, or impervious surfaces).

Impervious Surface

A surface (pavement, rooftops, parking lots) that blocks infiltration, increasing runoff, flood peaks, and pollutant transport while decreasing groundwater recharge.

Stream Flashiness

Rapid rises and falls in streamflow (often in urbanized watersheds) caused by quick runoff delivery and reduced infiltration/storage.

Point Source Pollution

Pollution from a single, identifiable discharge location (such as a pipe); typically easier to regulate because the source is clear.

Nonpoint Source Pollution

Diffuse pollution from many locations across a landscape (e.g., fertilizer runoff); strongly tied to storms, runoff, and land use within a watershed.

First Flush

A pulse of highly concentrated runoff pollution that often occurs during the first major storm after a dry period, washing accumulated contaminants into waterways.

Eutrophication

Nutrient enrichment (often nitrogen and phosphorus) that fuels algal blooms; decomposition of algae can lower dissolved oxygen and harm aquatic life.

Sediment Pollution

Excess soil particles in water from erosion that reduce clarity, smother habitats/eggs, and can carry attached pollutants like some nutrients and pesticides.

Riparian Buffer

A vegetated strip along streams that traps sediment, absorbs nutrients, stabilizes banks, and shades/cools water (supporting higher dissolved oxygen).

Green Infrastructure

Urban practices (rain gardens, permeable pavement, green roofs) that increase infiltration and reduce runoff and pollution compared with conventional stormwater systems.

Trade Winds

Persistent tropical winds that usually blow east-to-west along the equator; they push warm surface water westward in the Pacific under neutral conditions.

Upwelling

The rise of cold, nutrient-rich deep water to the surface (notably off western South America in neutral conditions), supporting phytoplankton and productive fisheries.

Thermocline

The boundary layer separating warm surface water from colder deep water; its depth affects how easily nutrient-rich deep water can upwell.

El Niño–Southern Oscillation (ENSO)

A natural climate pattern involving coupled ocean-atmosphere changes in the tropical Pacific that shifts precipitation, storm tracks, drought/flood risk, and ocean productivity.

El Niño

The warm phase of ENSO: trade winds weaken, warm surface water spreads east, the eastern Pacific thermocline deepens, upwelling weakens, and rainfall patterns shift.

La Niña

The cool phase of ENSO: trade winds strengthen, warm water is pushed farther west, the eastern Pacific thermocline shoals, upwelling strengthens, and rainfall patterns shift.