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On the Trails: West side of Mendenhall Lake

Posted: November 9, 2012 - 1:00am
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Water boatmen row around by using their hind legs. The front legs are small and used for feeding.  Photo by Bob Armstrong
Photo by Bob Armstrong
Water boatmen row around by using their hind legs. The front legs are small and used for feeding.

Late October: to the amazement of most Juneau residents, we had a week or more of clear skies, bright sun, and very nippy temperatures (nighttime temperatures of eleven degrees at my house!). On one of these fine days two friends and I sauntered along the beach on the west side of Mendenhall Lake. The cold had hit the upper elevations a few weeks earlier, so water levels in the lake were low and the little streams that come into the west side of the lake were tiny riffles, easy to hop over.

Partway up the beach, we happened to turn around and saw five swans flying over our route. A splashy ruckus farther back down the beach demarcated the take-off point for seven more swans. These caught up with the first group, and all twelve circled up over the glacier and back downstream. A day later, and we couldn’t have seen them here, because ice then covered even the shallows of the lake. So we counted ourselves lucky.

Along the beach we noted several ancient-looking logs and stumps, embedded in the sand. These are trees that were flattened by the Mendenhall Glacier when it expanded during the Little Ice Age glaciation. According to local experts, some old timbers near the glacier terminus have been estimated to be 1,200 to 2,000 years old. Between the Pleistocene glaciation and the Little Ice Age, forests could grow in this area. But the advancing glacier destroyed that forest, and buried the dead trees in outwash sands as it retreated (which it began to do about 250 years ago). Many times I have walked past those old trees and never noticed them! Shame!

Our destination was the rocky bench on the south side of the rock peninsula across from the visitor center. As on most of our best little explorations, we had no particular objective, except simply to see what we could find. Some small ponds on the rocky bench provided entertainment, although most were covered by a film of ice.

In the shallows, in the narrow strip of open water between the shore and the ice, we captured a very small larva, maybe that of a cranefly. Cranefly larva can grow to be nice, big, fat, juicy bird food, but this one had quite a way to go. The long-legged, flying adults are bird food too.

We also captured a small larva of a predaceous diving beetle. These larvae grab their prey with mighty pincers, and they eat almost anything that moves. Our captured larva was less than an inch long, but these beetle larvae can eventually reach lengths of several inches. That’s big enough to be worth catching by a kingfisher, and big enough for the pincers to be felt sharply by a human captor!

Predaceous diving beetles have a weird life history. Both larva and adult are aquatic, but adult females lay eggs on land or inside of plants. The larvae go into the water, but when they mature, they move back onto land to pupate, while they transform into adults. It’s not unusual for species with aquatic larvae to have terrestrial adults, but it is odd for an aquatic insect to use land twice in its life history.

Scooting around under the ice were water boatmen. They swim by using their hind legs as oars. Like the diving beetles and several other aquatic insects, they are air-breathers. So, in this way too, these insects have not entirely left the land.

At the side of a small waterfall we watched a pair of little flies, possibly chironomid midges, engaged (apparently) in the preliminaries to mating. They took a long time about it, and we moved on before they accomplished it.

As we climbed back up the hill to the main West Glacier Trail, we noted old beaver works high on the slope. Several trees had, long ago, been chewed or cut down. At first thought, it seemed odd for beavers to clamber so far up a hill, when trees seemed to be available closer to shore. Then we recalled that a couple of years ago, a beaver lived for a time above the Eaglecrest lodge, building small dams and a house. So beavers do what they need to do, in order to survive.

• Mary F. Willson is a retired professor of ecology.

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