Hydrohistorian
Lesson B3 · The River · hydrology
The season is the system
Colorado rivers keep two calendars. Nature’s: snow, melt, recession. And the ditch rider’s: headgates opening in April, the call coming on as the river drops, everything closing by November. The second calendar is carved into the first so deeply that you cannot read one without the other.
Watch a century of it
The evolution animation plays a basin’s whole networked life — every gage and structure reporting, month by month, from the 1880s forward. A narration for your first watch:
Watch spring: structures light up almost together in April–May — that synchrony is law as much as weather, priorities coming into season. Watch fall: the shutdown is even sharper — the annual close of irrigation, the rhythm an earlier version of this site hid by only animating April–October. Watch the decades: the network itself grows — more structures, more storage; the river’s plumbing thickening across a century. And winter’s odd kinks are often the instruments, not the river: ice makes winter measurement hard, and the animation notes say so.
Why the season matters legally
Seniority is seasonal too. A right that is comfortably in priority in June may be called out by August every dry year — the week-by-week climatology in lesson A1 exists precisely because annual summaries hide the rhythm that decides whether late-season crops get watered. In Colorado, when you are senior matters as much as whether.
What this does not let you conclude
The animation shows reported diversions and flows, not consumptive use; early decades are only as complete as their record-keeping, and a structure appearing mid-century may mark the start of its records rather than of its water use. 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.