Alder Reservoir

As of August 25, 2026, Alder Reservoir is filled to 83.2% of its total capacity, holding roughly 220 hm³ of water out of a maximum 265 hm³. At this level the reservoir is high.

Over the most recent week on record the fill level has held broadly steady (-1.3 pts), and it is 17.6 points above its level on the same date a year earlier.

Across the readings we hold for Alder Reservoir — 1425 data points between October 1, 2022 and August 25, 2026 — the level has ranged from a low of 41.6% (October 24, 2025) to a high of 98.3% (June 15, 2024). On average the reservoir is fullest around June and at its lowest around October.

It is operated by USGS primarily for multiple purposes. It lies within the Washington basin at 46.802°N, 122.311°W.