Water that crossed the divide

A tunnel's gage is a ledger, not a river

A stream gage on a creek measures what nature and upstream users left. A gage at a tunnel portal measures a decision: how much water an operator chose to move that day, inside its decreed limits. The Roberts Tunnel record (ROBTUNCO) rises when Denver needs more Blue River water and falls when it does not — so a rising “trend” there says something about demand and operations, never about snowpack. That is why basin trend pages here mark transbasin gages with ⇆ and keep them out of the hydrology headline (lesson B4 explains the headline's rules).

Imported water plays by different rules

Water native to a river must be shared with the river: divert it, use it, and the unconsumed share returns to the stream where juniors and downstream states have rights in its pattern (lesson B2). Water imported from another basin — the law calls it foreign or developed water — owes the receiving river nothing. The importer may capture, store, re-divert and reuse it to extinction, because no one on the receiving river ever had a right to water that would not naturally be there. That single doctrine is why cities pay for tunnels: an acre-foot imported is worth more than an acre-foot of native right, because it can be used more than once.

The flip side sits with the basin of origin: the exported water is simply gone from its home river — no return flows, no reuse, and decades of West Slope–Front Range politics follow from exactly that.

The registry, live

The facilities Hydrohistorian treats as transbasin — hand-curated, never guessed from names — with their gaged records where DWR meters the portal. The full reference, with a map, is at /transbasin.

What this does not let you conclude

The registry is curated and deliberately incomplete: name tokens like “tunnel” also belong to mine drainage and municipal pipelines, so a facility appears here only once confirmed. A tunnel gage's volume is deliveries, not depletions — what the basin of origin lost also includes evaporation and operational spills the portal gage never sees. And reuse-to-extinction is the doctrine's shape, not a title search: what a specific importer may lawfully do with specific water is a decree question. 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.