What is the cause of the declining Colorado River supplies?

**Natural factors:** Sustained multi-decade drought (the Southwest "megadrought," worst in ~1,200 years per tree-ring reconstructions) and reduced Rocky Mountain snowpack — the river's dominant supply source. Rising regional temperatures increase evaporation, reduce snowpack-to-runoff efficiency (more sublimation, earlier/faster melt, drier soils absorbing more runoff before it reaches streams), and shrink the fraction of precipitation that becomes usable flow. These are climate-driven, not one-off weather anomalies.

**Human factors:** Structural over-allocation — the 1922 Colorado River Compact and subsequent apportionments allocated more water (~16.5+ maf/yr across the basin) than the river's actual long-term average flow (closer to 12–13 maf/yr even before recent drought), so demand has exceeded supply by design for decades. Agricultural use (roughly 70-80% of consumptive use, dominated by irrigation) is the largest draw, with municipal/urban growth in the Southwest adding pressure. Reservoir evaporation losses (from Powell and Mead's large surface areas) and conveyance losses compound the deficit.

**Infrastructure/reservoir state:** Lake Mead and Lake Powell — the two largest storage reservoirs, built to buffer variability — have both dropped to historically low levels (each has approached "dead pool" thresholds where Hoover/Glen Canyon Dam hydropower and downstream releases become jeopardized). The buffer capacity that used to absorb bad years has been drawn down by consecutive dry years plus structural overdraw, leaving the system with far less resilience against further shortfall. Dams and canals themselves are aging but functional; the acute problem is depleted storage, not physical infrastructure failure.

**Bottom line — the cause:** It's a compounding of three things acting together, not a single cause: (1) climate-driven reduction in snowpack/runoff and higher evaporative losses, (2) legally-entrenched over-allocation where paper water rights exceed real river flow, and (3) reservoir storage drawn down to near-critical levels, removing the historical cushion against drought years. Natural drought set the stage; human allocation and demand patterns turned a bad hydrological stretch into a structural supply crisis.