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What's Happening on the Hudson River in September

monarch butterflyJust when Hudson Valley residents have had all they can take of the haze, heat, and humidity of summer, along comes September. The ninth month will have its hot spells, but the sun sets a few minutes earlier each evening, a reminder that we're in transition to the clear, chilly days of October.

For hikers, this may be the single best month to visit the High Peaks of the Adirondacks. The crowds of summer hikers have left, the days are cooler, and you may actually be able to carry enough water for an overnighter. The crisp nights treat us to a star show, and coyotes and owls serenade us to sleep.

September days can be bathed in crystal-clear, cool Canadian air or in humid, warm air with downpours of tropical intensity. This dynamic variation is mirrored in the ebb and flood of life on the Hudson, giving September great appeal for river-watchers. In most years the salt front is as far upriver as it is likely to be, while eddies of warm water are still spinning off the Gulf Stream and into the New York bight.

As a result, this is a time when seine and trawl nets capture fish from distant climes. To the list of tropical strays mentioned last month, fishermen might add Spanish mackerel, scup (porgy), lookdown, and oyster toadfish. Yet if net haulers can take a moment to look up from their work, they will notice creatures from more temperate climes heading south.

Northerly breezes that follow September cold fronts carry along many fall migrants. Most noticeable, delicate monarch butterflies flutter down the valley, headed to wintering locations in the remnant rain forests of central Mexico's mountains, 2700 miles away. Common green darner dragonflies are also heading south, but less is known of their eventual destination. Migratory birds follow these same thoroughfares, and also dictate their movements according to the winds, but they, unlike the insects, will make the return trip as well. Flocks of songbirds and shorebirds will queue up on peninsulas along the Hudson's east bank, waiting for a good tailwind to cross the river and head down the valley.

When the wind is right a nearly continuous flight of raptors, birds of prey, wings overhead. The most commonly seen will be the broad-winged hawk. This species migrates south by soaring nearly effortlessly along mountain ridgelines. They ride up thermals - columns of warm, rising air - and once high enough, glide off south until they need to circle upwards again. As the day goes on, their numbers build to the point where dozens and even hundreds of hawks might be swirling upwards in one thermal - an impressive phenomenon birders call a "kettle." Birders return to favorite hot spots to watch this migration every fall. The ridge lines of the Hudson Highlands, Storm King, Breakneck Ridge, Bear Mountain, Anthony's Nose, Hook Mountain, and the Shawangunk ridge offer excellent vantages for watching raptors hitching rides on the northwest winds. In addition to broad-winged hawks, other birds of prey passing the lookout include eagles, osprey, harriers, accipiters, and falcons.

Autumn migrations are not limited to the air, although those are the most obvious. Beneath the surface of the river a procession of young-of-the-year fishes continue their migration seaward. Hundreds of thousands of herring, shad, and striped bass, born in springtime, nurtured all summer by the estuary, will spend the next several years growing in the North Atlantic. One day, many of them will return to their natal river to spawn.

With the Hudson teeming with fish, their predators are plentiful as well. Although we see marine mammals along the river in every month, most commonly harbor and harp seals, September seems to have more than its share. Seals are liable to show up on almost any dock or jetty along the Hudson's tidewater reach. Viewers from Hudson Line trains might also see a couple of dozen great blue herons, also enjoying this period of plenty. Large fish are also following their prey into the lower, brackish estuary from the New York Bight. Large "alligator blues" are in the Tappan Zee, and "schoolie" stripers (5-15 pounds) are commonly taken by anglers from Staten Island to Croton Bay.

As the days grow shorter and cooler, the warmth of the river, absorbed over a long summer, is slow to leave. Those who would like summer to linger as long as possible find solace in the tepid shallows throughout September, past the autumnal equinox. The warm water and cool nighttime air temperatures create morning fogs along the Hudson - lovely to look down upon from the Highlands peaks, but hazardous to rush hour motorists. This is the month for a long river walk, accompanied by butterflies and songbirds, hawks and falcons, and a sense that the river is winding down for another year.

Crimson-splashed red maples in low-lying swampy areas where cold air collects prelude the season's grand finale. But there is time enough for that in the months to come. For now, for this month, we will shout encouragement to the monarchs and revel in the glow of pastel sunsets.

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