Seventeen mountain lakes, watched from space. Most of them will never see a sampling boat, so the satellites do the looking: what colour the water is, how green it's getting, when the ice goes out and how warm the surface runs.
Colour. Every lake gets a number on the Forel-Ule scale, the 21 vials of coloured water limnologists have held up against lakes since the 1890s. Low numbers are deep blue and clear; higher numbers are greener, then browner.
Each lake against itself. Waldo and Crater Lake are naturally bluer than a reservoir will ever be, so no lake is held to a single number. This summer is compared with the summers before it, and a lake is flagged only when it is clearly greener, warmer or more chlorophyll-heavy than its own normal.
Click any lake for its satellite picture, its recent summers and what the ground sampling says, where there is any.
Every few days the European Sentinel-2 satellites photograph each lake at 10 to 20 metres. For each clear pass, ALPINE-WATCH reads only the open water, keeping 40 metres in from the shore so beaches and shallows don't leak in. Big lakes are sampled inside a 3 km circle so it's the same water every time.
Colour comes from the visible bands, converted to a hue angle and a Forel-Ule number with the published Sentinel-2 coefficients (Van der Woerd & Wernand), the same ones used in ESA's SNAP toolbox.
ESA changed how it processes Sentinel-2 in January 2022, and over dark water that alone shifts the colour, so colour and the chlorophyll index are compared from 2022 on. Temperature and ice-out use the full record.
NDCI compares the red-edge band with the red band, which rises as algae thicken. It's good at spotting a lake getting greener, but in very clear lakes it can't be turned into micrograms per litre honestly, so it's shown as an index and judged only against the lake's own past.
Sentinel-2 classifies ice and snow, so the spring day each lake opens up can be pinned to within a few days. Landsat 8 and 9 measure surface temperature from their thermal band, every 8 to 16 days since 2013. Summer temperature here is the July and August median.
Small, dark lakes in deep granite basins, like Kibbie and Many Island: the standard atmospheric correction can push their blue and green to zero, and Landsat's 100 m thermal pixels can't fit inside them without catching hot rock, so for those it's mostly ice-out. Also lakes smaller than a few hectares, the colour of water under cloud or smoke, anything below the surface, and toxins. Diablo Lake's turquoise is glacial flour, so its colour tracks the glaciers more than algae. Mono Lake is salty, and its colour follows brine shrimp and algae on its own rules.
A flag here means "worth a closer look," not a diagnosis.