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.
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The relationship between monarchs and milkweed is often described as simple, but the timing makes it complicated. Monarchs that overwintered in Mexico began moving north in spring, breeding as they went. The females arriving here now are likely the daughters or granddaughters of those overwintering adults, raised on milkweed further south and then continuing the northward movement. They need milkweed to be available when they arrive, which means the plant's flowering schedule and the insects' migration schedule have to align. In a year with a warm early spring, milkweed may flower earlier than usual. In a cool year, it may lag. The monarchs adjust somewhat, but the window is not unlimited.
Somewhere nearby, if the light is good and the air is warm, a monarch may be working the edges of a sunny patch right now, pausing on each milkweed plant just long enough to check it before moving on. The orange of the butterfly and the orange of the milkweed flowers are close enough in color that the two can be hard to separate at a distance. Watch for movement: the slight, deliberate shift from stem to stem that distinguishes an egg-laying female from a butterfly simply feeding.
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in the genus
Hexagenia
were functionally eliminated from southern Lake Michigan by the mid-twentieth century — not by overharvest or habitat loss in the conventional sense, but by suffocation. Untreated industrial and municipal waste flowing into the lake consumed the dissolved oxygen in bottom sediments, and giant mayfly nymphs, which spend up to two years burrowing in fine lakebed muds and filtering organic matter, simply could not survive the resulting dead zones. Their disappearance was so complete that it registered as a threshold collapse, the kind ecologists use as a before-and-after marker for an entire system's health. The passage of the Clean Water Act in 1972 and the subsequent buildout of wastewater treatment infrastructure began to reverse that collapse, and by the 1990s and early 2000s, researchers were documenting the first tentative recolonization pulses moving southward through the lake basin.
What makes this summer's sightings complicated — and why they cannot be read as simple victory — is that the watershed feeding this shoreline remains under active stress. The
MILWAUKEE METRO SEW DIST COMBI
combined sewer system is currently in significant non-compliance with its Clean Water Act permit, meaning that during heavy rain events, untreated sewage and stormwater are still being discharged directly into receiving waters. The Menomonee River, one of the two main river systems draining into this shoreline stretch, carries an official impairment listing — a formal acknowledgment that it is not meeting basic water quality standards. Giant mayfly nymphs are exquisitely sensitive to low dissolved oxygen and fine-sediment quality; the same conditions that once wiped the species out regionally are, in a less acute form, still being generated by overflow events today.
The giant mayflies have returned to a system that has not yet returned to them.
That asymmetry is the defining ecological tension of this moment on the riverfront. Giant mayflies are recolonizing the margins of a recovery that remains incomplete, and whether these populations persist and expand or contract back will be determined in large part by what happens in the sediment over the next several years — how often anoxic pulses reach the river bottom, how much fine organic waste accumulates, and how reliably cool, oxygenated conditions hold through summer. The species is doing the work of a bioindicator whether we ask it to or not.
The broader ecoregional context matters here too. This riverfront sits within the
Lake Michigan Lacustrine Clay Plain
, a landscape shaped by glacial lake clays that drain slowly, concentrate runoff, and amplify the downstream effects of urban impervious surfaces during storm events. That geology is not incidental: it is why combined sewer overflows here carry such ecological weight, and why summer storm intensification — already a documented trend in the Great Lakes region — poses a compounding threat. As the season's heavier rains arrive, the sightings record here will become a real-time legibility test for how much the system can absorb before the recovery signal at the riverfront dims again. Watch the emergence windows carefully through July.
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Late August through September is when the young fish leave, and they leave in pulses. The Maine Department of Marine Resources describes fall rainstorms as the trigger that sends them out of the lake outlets in dense schools toward Benton Falls and the sea. A dry September removes that cue. Maine fish-passage programs name drought as one of the main hazards for exactly this size class at exactly this season, because juveniles that never get the signal stay in warm, food-limited lake water longer, and juveniles that do move through low water bunch up at every constriction, where eels, herons, mink, otters and eagles already hunt them each fall.
Whether this year's cohort is running on schedule is not something anyone has counted; the fish lift tallies adults in spring, not juveniles in fall. What can be said is that the county crossed into severe drought in the same week the young alewives would normally be starting to go. Anyone who wants to see the run should watch the lake outlets and the tailrace at Burnham after the next real rain, and look for a moving raft of finger-length fish, not scattered individuals.
Sebasticook Regional Land Trust
, which started as Friends of Unity Wetlands and later absorbed the Sebasticook River Watershed Association, runs paddles above the Burnham dam and bird walks at Unity Pond, and its Moulton's Mill Preserve in Unity sits on Twenty-five Mile Stream, one of the feeders of this system.
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