Reading a hydrograph

The one reading habit that matters

Never read a flow against “the average.” Read it against the distribution for that day. Colorado rivers are so variable that the mean year is a statistical fiction — half of Junes may be double the other half. That is why this site’s charts draw percentile envelopes: today’s flow against the 10th–90th percentile band for this calendar day over the whole period of record. “62% of median for late August” is a statement with teeth; “below average” is barely a statement at all.

Three shapes to know on sight: the snowmelt peak (its timing moves weeks between years — watch the x-position, not just the height); the irrigation notch (an abrupt summer drop that is headgates, not hydrology — the river stepping down as calls come on); and winter baseflow (fed by groundwater, which is why wells and rivers are one system). Ice can garble winter measurements — our animations say so when it matters.

Read a real one

Open any basin’s Trends tab and click a gage: the chart that opens is the exact series its statistics were computed from, drawn with the envelope described above. The basin page’s date picker reconstructs any historical day — pick a June day in a drought year and a wet year and watch the same river be two different rivers.

What this does not let you conclude

A gage measures the river at one point — not what was diverted above it (a “low” reading may be administration, not drought), and pre-1950 daily records can carry digitization quirks our USGS cross-check flags per basin. 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.