Sevier River

The Sevier River is a river, not a lake: it carries water from its headwaters to the Great Salt Lake. The model is a flow balance for each water year (Oct 1 – Sep 30): flow to lake = inflow − diversions − residual.

Inflow is the measured total at Sevier River near Juab (USGS 10219000), split into main stem and tributaries — the sliders on the left. Diversions are the human-controlled takes in the Box Elder Valley (about 0.09 MAF/yr at full build) — the sliders on the right. Residual is a calibrated fraction of inflow covering river evaporation, seepage, storage change, and the gap between the last gauge and the lake (there is no gauge at the mouth).

Because diversions are governed by water-right priority, not a fixed share of flow, the take scales with available flow up to 90% of inflow. In a wet year the river carries plenty of water, so the fixed diversions take a small fraction and most of the flow reaches the lake. In a dry year the same diversions take a much larger fraction — up to that cap — so far less reaches the lake. The “Where the water goes” breakdown in the what-if card shows exactly where each year’s inflow ends up.

• The selected year is labeled. The default is the last complete water year. The in-progress year is available in the year picker but flagged as partial — its low total reflects only the months recorded so far, not a full year.
• No gauge at the mouth. The flow to the lake is a computed residual (inflow minus diversions minus residual losses), not a direct measurement.
• Diversion volumes are estimates. The precise year-by-year diversions live in the Utah DWR diversion database, a JavaScript app the pipeline cannot read. The figures here are grounded in the measured flow loss from Corinne to the below-O-Line gauge and published basin totals, but they are not the official record.
• The Corinne gauge is pre-diversion. It reads the river before the Box Elder Valley takes, so the inflow is the water available to divert, not what reaches the lake.
• Tributaries are a fixed split. The main-stem / tributary split is a documented proportion of the Corinne total, not independently gauged (the tributary gauges have no continuous record).

Every figure on this page traces back to a public source. The measured inflow comes from the USGS Water Data OGC API (long-term mean 0.17 MAF/yr); the diversion volumes are documented estimates.

• Inflow: Sevier River near Juab — USGS 10219000 USGS page
• Sevier Valley irrigation: Estimate. The Sevier Valley irrigation districts (Richfield, Circleville, etc.; the Richfield Irrigation Canal dates to 1860) are the largest diversion. Sized so the total diversion + residual matches the published figure that only ~32,900 AF of the ~823,000 AF basin runoff reaches Sevier Lake.
• East Fork / Sanpete Valley ditches: Estimate. Smaller ditches and canals on the East Fork and Sanpete Valley stretch (e.g. the Tropic Ditch, which diverts 15 miles from the East Fork to Tropic).