Great Salt Lake "What-If" Water-Level Dashboard

Pick a water year, then drag a river's inflow slider to see how the lake's water balance would have changed. Static page, no backend — data is pre-fetched USGS JSON.

Inflows

Water year

Total inflow
—
actual — · —

Selected year

Lake level, Oct 1
—

Surface area
—

What-if: — end level (Sep 30)

What-if end
—

Actual end
—

Δ vs actual
—

5-year projection

Start year

Projected end
—

BAU end
—

Δ vs BAU
—

How this works

The Great Salt Lake is a terminal lake: it has no outflow, so its level rises with inflow and falls with evaporation. For each water year (Oct 1 – Sep 30) the model steps the level once — Δh = I / A − e — where I is the total inflow set by the sliders (million acre-ft, MAF), A is the lake's surface area at the current level from the published elevation–area curve (a bigger lake rises less from the same inflow), and e is the annual evaporation depth (ft/yr).

The 5-year projection sustains the slider inflows for five water years from a chosen start year and compares them against business-as-usual (long-term mean inflow); the main chart overlays both on the observed 1847–2026 record, with the 4,200 ft historic-average level and the 2022 low marked.

Limitations

Evaporation is a calibrated residual, not a measurement. e is fitted from the observed level change and the measured inflow, so it absorbs evaporation plus groundwater, diversions, and unmeasured streams — and can even go negative in very wet years.
Gauges sit upstream of diversions. The river gauges (e.g. Corinne on the Bear) read flow before major diversions take water away, so measured inflow overstates what actually reaches the lake. The calibrated residual absorbs that offset for historical years, but what-if scenarios inherit the bias.
Flat-lake approximation. Each annual step spreads the year's inflow over the surface area at the current level; the area change within the year and the basin's 3-D shape are ignored. Fine at annual resolution, but a coarse representation of the lake.
Data gaps. No Bear River gauge data 1957–1962 (its slider defaults to 0 and the residual absorbs the missing flow); 2026 is a partial year; the 1847–1948 level record is monthly.
Fit quality. Run on actual inflows, the model reproduces the observed 1949–2025 level within ~0.2 ft RMSE, with the 1986 high and the 2022 low within ~1.2 ft.