Why the Great Basin Is an Inland Drainage System

The **Great Basin** is North America’s **largest endorheic (internally draining) region**—~400,000 km² where **no river reaches the ocean**. Here’s why.

1. The Rain Shadow Wall: Sierra Nevada

• **Prevailing westerlies** carry Pacific moisture.
• **Orographic lift** on windward slopes → **1,500–2,500 mm/yr** rain/snow (Sierra west slope).
• **Air descends** leeward → **adiabatic warming, drying** → **rain shadow** over the Basin.

2. Basin & Range Topography Traps Water

• **160+ N-S mountain ranges** block lateral flow.
• **90+ basins** are **hydrologic bowls**—water flows *down* to the lowest point (playa, lake, sink).
• **No master stream** integrates the region—each basin is a closed hydrologic unit.

3. Aridity Prevents Overflow

• Potential evapotranspiration (PET) >> Precipitation. PET ~1,500 mm/yr; Precip ~150–300 mm/yr.
• Even wet years (e.g., 1983, 2023) only **temporarily** connect basins (e.g., Sevier → Great Salt Lake). No permanent outlet forms.

4. Terminal Lakes & Playas = Evaporation Sumps

5. Geologic Youth & Tectonics

• **Basin & Range extension** (17 Ma–present) creates new basins faster than streams can integrate.
• **No coastal plain**—the Sierra Nevada / Wasatch walls are the continental edge.

Consequences

• Saline lakes – unique ecosystems (brine shrimp, migratory birds).
• Groundwater mining – basins as "bathtubs" with no drain; pumping = permanent drawdown.
• Dust storms – exposed playas (Owens Lake, Sevier Lake) → PM10 pollution.
• Endemism – isolated aquatic habitats → **relict species** (Desert pupfish, springsnails).

The Great Basin isn’t "missing" an outlet—it’s a **hydrologic continent unto itself**, where water’s journey ends in salt and sky.