Summer upwelling plunges nearshore temperatures ten degrees

July 23, 2026

Nearshore water temperatures have dropped sharply to 16.5°C this week — a plunge of more than ten degrees from the dangerous highs of a month ago, when tributary streams were pushing the thermal limits for cold-water fish. The shift is driven by a classic summer upwelling: persistent southwesterly winds push the warm surface layer offshore, pulling cold, deep water up along the shoreline in its place. The change can be dramatic — sometimes dropping temperatures ten degrees in a single day.

strandings documented from Milwaukee Bay south to Wind Point through June fit this pattern closely. These introduced forage fish are acutely vulnerable to the thermal shock that upwelling delivers—the phenomenon was implicated in the catastrophic beach die-offs of the 1960s that ultimately prompted the stocking of Pacific salmon as a biological check on their population. For stressed by heat in tributary streams all summer, the cold plume offers a brief reprieve — though it is temporary, a seasonal rhythm rather than a rescue.

Data from USGS and iNaturalist

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