The Lake Champlain Watershed

Each dot is the first time anyone recorded zebra mussels there — not when they arrived, and not how many. The color and the year slider both run on that first-record date. Most of the surveying happened between 1996 and 1999, so the dates trace where people looked as much as where the animal went. Lake Champlain only.
USGS Nonindigenous Aquatic Species database.

The lake's own shape, from forty-one closed depth contours at ten-foot (3 m) intervals — also used on the budget page. Gold is shallow, violet is deep — the same scale, keyed to the same depths in feet, that colors the lake bed on the front page.

0 ft 400 ft

The lake has two very different parts: a trench four hundred feet (122 m) deep down the main lake, and a northern half that is shallow throughout. That difference governs which end blooms, where a thermocline can set up at all, and the period of the seiche.

Every place the Vermont Department of Health has sampled for cyanobacteria, colored by how often it was under an alert rather than by how many samples were taken there. Thirteen seasons, 2012 to 2024. A pale dot is a place that has been checked and has mostly been clear; a red one is under an alert a quarter of the time or more. The water itself is shaded by the same measure over each of the eight segments the program reports against — those outlines were traced for this site's water budget, by flooding outward through the lake from the monitoring sites themselves. They do not tile the lake exactly; the darker water between them is water the program does not report on.

never half the visits

Vermont Department of Health cyanobacteria tracker · the whole record ↗

The fields themselves: pasture and hay and cultivated crops in the United States, cropland in Quebec. The two classes are kept apart because they are two different land uses that contribute phosphorus differently. Click a river for its own basin's farmed share, which is still the figure the phosphorus work uses.

pasture / hay crops cropland, Quebec

Two national datasets meet at the border, and they classify land differently: the American one separates pasture from crops, the Canadian one has a single cropland class, so Quebec appears in a single class. The most heavily farmed ground in this whole frame is on the Quebec side of it — the Missisquoi headwaters and the Pike — draining into Missisquoi Bay, which is the bay that closes for cyanobacteria more often than any other.
USGS/MRLC National Land Cover Database 2021 · Natural Resources Canada, 2020 Land Cover of Canada.

Lake surface: Sentinel-2, 4 August 2026, cloud-free. Water and land are stretched separately; the colors are the satellite's own red, green and blue bands, not false color.

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Where every layer comes from

All of it is public. Three of the layers carry years USGS has not published — those are marked ESTIMATED, and the method and its check against the published years are given with them. Nothing else on this map is computed by this site except stream weight and the segment outlines, both noted below.

The base map

Streams, ponds, the lake, the divide: USGS National Hydrography Dataset, hydrologic unit 0430. The basin is units 04300101–08; 04300109 is the Richelieu, which is the way out, so it is drawn with the water beyond the divide rather than with the catchment.
Land beyond the divide: The same dataset, units 0202 (Upper Hudson) and 0108 (Connecticut) — the streams in this frame whose water ends up somewhere other than Lake Champlain.
The Quebec side: NRCan CanVec hydrographic features, 1:250 000, which USGS does not map.
Terrain and relief: USGS 3DEP 1 arc-second elevation, nine one-degree tiles, resampled to about 70 m (230 feet) and hillshaded here.
Stream line weight: Not a source: upstream accumulated stream length, computed here by walking NHD's own flow table. NHD ships a stream-order attribute, but it has no rows at all for the Ausable, the Saranac or the lakeshore tributaries, which drew the whole Adirondack side as hairlines.

Phosphorus, road salt, farmland and discharge, by river

Phosphorus, 1990–2011: Medalie, USGS Scientific Investigations Report 2013–5021, appendix 1, as published. A second release carries it to 2014 (OFR 2016–1200).
Phosphorus, 2015–2024: ESTIMATED Fitted here by WRTDS (Hirsch, Moyer and Archfield, 2010), the method USGS used, from the same three inputs: samples from the Lake Champlain Long-Term Monitoring Program via the Water Quality Portal, USGS daily discharge at the seventeen U.S. gauges, and the Quebec gauges below. Run against the twenty years USGS did publish, it reproduces their basin total within 5.7 per cent in the worst year, median 2.5, and 4.2 per cent per river. If that stopped being true the estimated years should come off this map.
Road salt (chloride): ESTIMATED after 2011, the same way, from the portal's chloride record for the same eighteen rivers. Its estimates match the published years more closely than the phosphorus estimates do: 2.9 per cent per river, 9.1 in the worst basin year.
Farmland: USGS SIR 2013–5021, appendix 6 — the basin characteristics that report correlates its loads against.
Discharge: USGS daily values for the U.S. gauges; Quebec's Centre d'expertise hydrique stations 030420 and 030424 for the Pike, spliced at the 2012 gauge move by the ratio of their flows over the eleven years both ran.

Calcium, zebra mussels and blooms

Calcium: Lake Champlain Long-Term Monitoring Program via the Water Quality Portal; the median of every calcium sample at each of the eighteen tributary stations.
Zebra mussels: USGS Nonindigenous Aquatic Species database, pulled 2 September 2026: 838 records at 135 places, 1993 to 2021. Only the first record at each place is drawn, and only Lake Champlain — records in Lake George, Bomoseen, Glen Lake, the Richelieu and the Chambly canal are other water.
Blooms: Vermont Department of Health cyanobacteria tracker: thirteen season-summary spreadsheets, 2012 to 2024, about 27,000 reports filed by trained volunteers and state staff. Site coordinates come from the tracker's own feature service. Rates are shown rather than counts, because reporting effort grew five-fold over those thirteen years.
Segment outlines: Not a source: the monitoring program's own segment names, grown outward from its sampling sites by flooding through the water for this site's water budget, so a bay's label cannot leak across a headland.

Bathymetry and the satellite view

Bathymetry: Vermont Center for Geographic Information isobaths digitized from NOAA 1:40 000 charts — forty-one closed contours at ten-foot (3 m) intervals, as GeoJSON.
Satellite view: Sentinel-2 L2A, 4 August 2026, read band by band out of the public cloud-optimized GeoTIFFs. Water and land are stretched separately — water sits at a few per cent reflectance against forty for a hayfield, and one stretch cannot serve both — but the colors are the satellite's own red, green and blue.