Hydrohistorian
Lesson A3 · The Line · prior appropriation
Why downstream can beat upstream
The most counterintuitive rule in the system: a senior right thirty miles downstream can shut off an upstream junior. Water administration ignores the map and reads the calendar — when a senior cannot get its water, the callA senior water right demanding its water, which requires everyone junior on that stream to stop diverting until it is satisfied. it places curtails juniors by date, wherever they sit. Upstream, downstream: the river's geometry decides who can physically be curtailed to help; seniority decides who must.
Place a call on Clear Creek
Drag the year. You are setting a hypothetical call at that priority: every decreed right in the district junior to it would owe curtailment. Computed live from the district’s real rights index.
What you are watching
An 1870 call idles almost the whole district; a 1990 call idles almost nobody. Between those ends lives the entire economics of Colorado water: the date on the decree, not the size of the ditch or its position on the river, prices its reliability. This is why an 1861 right on a small ditch can be worth more per acre-foot than a 1950 right on a canal ten times its size — and why lesson A2’s paper water gets more papery the more junior the right.
One geometric honesty note: a real call administers the reach that can physically deliver water to the caller — broadly, juniors upstream of the calling headgate on the same source. The simulation above is seniority arithmetic across the whole district and says so in its own caveat; it answers “who is junior to this date,” which is the lesson, not “who would the commissioner curtail today,” which is administration.
What this does not let you conclude
This is a what-if over decreed rights, not a reconstruction of any actual call: no reach topology, no futile-callA call the division engineer declines to enforce because shutting off junior users would not actually deliver water to the caller. judgment, no plans or exchanges — and conditional rights, abandoned rights and changed rights all complicate a real cascade. 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.
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