Where water goes between two gages

The ledger between two gages

Walk downstream and keep books: out at every headgate (the diversion records from lesson C1); in at every tributary confluence; back in as return flows — the unconsumed share of upstream diversions seeping home days or weeks later; and in or out through the riverbed itself, where a reach gains from groundwater or loses to it. What remains when the books do not close is either measurement error or that last, invisible exchange — and distinguishing those two is much of river science.

Return flows deserve the emphasis: on heavily-worked rivers like the South Platte, the same water is diverted, partly returned, and re-diverted several times between the mountains and the state line. That reuse is why the river supports far more decreed demand than its native flow suggests — and why moving a right without protecting its return flows injures neighbors who never signed the deal (lesson E2).

See a river walked as a ledger

Every deployed basin’s longitudinal schematic is exactly this lesson drawn to scale: gages, diversions, returns and confluences placed at their river mile, walked downstream as a mass balance. Where the books do not close, the schematic says so rather than smoothing it over — the unclosed remainder is data, not embarrassment.

What this does not let you conclude

Reach gain/loss inferred from gage differences bundles together seepage, ungaged inflows, lagged returns and instrument error; separating them takes seepage runs and modeling that a two-gage subtraction cannot supply. Advisory reading aid on public data — never an administrative or legal conclusion, and never a substitute for DWR administration or an engineer’s analysis. Our methodology has not yet had outside expert review: how this is built.