Esla river · Duero Basin · dam built in 1987 · latest official reading: 6 October 2026.
Riaño is currently at 37.3 % of capacity, about normal for the time of year. The week-41 average across 36 years of records is 36.4 %, a difference of +0.9 points.
For context: a year ago it stood at 40.4 % and a month ago at 41.3 %. Against last week’s reading it is falling (−0.6 points).
Blue line: average monthly filling across the whole record, from 1989. The dot marks today’s reading, 37.3 % in October.
With 1,923 weekly readings since 1989, the average pattern of Riaño is clear: it peaks in May at 83.1 % and bottoms out in October at 36.6 %. That 46-point range makes it a reservoir with a very wide annual swing — almost half the pool between its best and worst month.
The highest reading in the whole series is 97.8 %, on 19 April 2016, and the lowest 0.0 %, on 5 December 1989. Comparing today against those two extremes says more than the bare percentage: 50 % is good news in a pool that rarely tops 60 %, and bad news in one that is usually full.
At 641 hm³ of capacity it is the third largest of the 35 dams the bulletin tracks in the Duero basin, holding 8.4 % of the whole demarcation’s capacity on its own. The basin as a whole is at 48.9 % today, so this reservoir is running below the basin as a whole.
The figures on this page come from the official MITECO reservoir archive, the same database behind Spain’s weekly hydrological bulletin. The bulletin covers every mainland reservoir above 5 hm³ of capacity and is published every Tuesday with Monday’s reading.
The latest reading here is from 6 October 2026, and the comparison series starts in 1989. Nothing is estimated, interpolated or forecast: the percentage is stored volume divided by the usable capacity the bulletin itself assigns to the dam. Historical averages are computed over the same calendar week in every previous year, the only fair comparison in a system with such marked seasonality.
See the whole demarcation, with all 35 dams the bulletin tracks: Duero Basin.
