What is the use, in Nature's economy, of these occasional floods in August?

August 25. 

 P. M. — To Hill by boat. Silvery cinquefoil now begins to show itself commonly again. Perhaps it is owing to the rain, spring like, which we have in August. 

I paddle directly across the meadow, the river is so high, and land east of the elm on the third or fourth row of potatoes. The water makes more show on the meadows than yesterday, though hardly so high, be cause the grass is more flatted down. 

I easily make my way amid the thin spires. Almost every stem which rises above the surface has a grasshopper or caterpillar upon it. Some have seven or eight grasshoppers, clinging to their masts, one close and directly above an other, like shipwrecked sailors, now the third or fourth day exposed. Whither shall they jump? It is a quarter of a mile to shore, and countless sharks lie in wait for them. They are so thick that they are like a crop which the grass bears; some stems are bent down by their weight. 

This flood affects other inhabitants of these fields than men; not only the owners of the grass, but its inhabitants much more. It drives them to their upper stories, — to take refuge in the rigging. Many that have taken an imprudent leap are seen struggling in the water. How much life is drowned out that inhabits about the roots of the meadow-grass! How many a family, perchance, of short-tailed meadow mice has had to scamper or swim! 

The river-meadow cranberries are covered deep. I can count them as they lie in dense beds a foot under water, so distinct and white, or just beginning to have a red cheek. They will probably be spoiled, and this  crop will fail. 

Potatoes, too, in the low land on which water has stood so long, will rot. 

The farmers commonly say that the spring floods, being of cold water, do not injure the grass like later ones when the water is warm, but I suspect it is not so much owing to the warmth of the water as to the age and condition of the grass and whatever else is exposed to them. They say that if you let the water rise and stand some time over the roots of trees in warm weather it will kill them. 

This, then, may be the value of these occasional freshets in August: they steam and kill the shrubs and trees which had crept into the river meadows, and so keep them open perpetually, which, perchance, the spring floods alone might not do. 

It is commonly supposed that our river meadows were much drier than now originally, or when the town was settled. They were probably drier before the dam was built at Billerica, but if they were much or at all drier than now originally, I ask what prevented their being converted into maple swamps? Maples, alders, birches, etc., are creeping into them quite fast on many sides at present. If they had been so dry as is supposed they would not have been open meadows. It seems to be true that high water in midsummer, when perchance the trees and shrubs are in a more tender state, kills them. 

It "steams" them, as it does the grass; and maybe the river thus asserts its rights, and possibly it would still to great extent, though the meadows should be considerably raised. Yet, I ask, why do maples, alders, etc., at present border the stream, though they do not spring up to any extent in the open meadow? Is it because the immediate bank is commonly more firm as well as higher (their seeds also are more liable to be caught there), and where it is low they are protected by willows and button-bushes, which can bear the flood? Not even willows and button-bushes prevail in the Great Meadows, — though many of the former, at least, spring up there, — except on the most elevated parts or hummocks.

The reason for this cannot be solely in the fact that the water stands over them there a part of the year, be cause they are still more exposed to the water in many places on the shore of the river where yet they thrive. Is it then owing to the soft character of the ground in the meadow and the ice tearing up the meadow so extensively? On the immediate bank of the river that kind of sod and soil is not commonly formed which the ice lifts up. Why is the black willow so strictly confined to the bank of the river? 

What is the use, in Nature's economy, of these occasional floods in August? Is it not partly to preserve the meadows open? 

Mr. Rice says that the brook just beyond his brother Israel's in Sudbury rises and runs out before the river, and then you will see the river running up the brook as fast as the brook ran down before. 

Apparently half the pads are now afloat, notwithstanding the depth of the water, but they are almost all white lily pads, the others being eaten and decayed. They have apparently lengthened their stems some what. They generally lie with more or less coil, prepared for a rise of the water, and perhaps the length of that coil shows pretty accurately to how great a rise they are ordinarily subject at this season. 

I was suggesting yesterday, as I have often before, that the town should provide a stone monument to be placed in the river, so as to be surrounded by water at its lowest stage, and a dozen feet high, so as to rise above it at its highest stage; on this feet and inches to be permanently marked; and it be made some one's duty to record each high or low stage of the water. Now, when we have a remarkable freshet, we cannot tell surely whether it is higher than the one thirty or sixty years ago or not. It would be not merely interesting, but often practically valuable, to know this. 

Reuben Rice was telling me to-night that the great freshet of two or three years ago came, according to his brother Israel, within two inches of one that occurred about forty years ago. I asked how he knew. He said that the former one took place early (February?), and the surface froze so that boys skated on it, and the ice marked a particular apple tree, girdled it, so that it is seen to this day. 

But we wish to speak more confidently than this allows. It is important when building a causeway, or a bridge, or a house even, in some situations, to know exactly how high the river has ever risen. It would need to be a very large stone or pile of stones, which the ice could not move or break. Perhaps one corner of a bridge abutment would do. 

Rice killed a woodchuck to-day that was shearing off his beans. He was very fat. 

I cross the meadows in the face of a thunder-storm rising very dark in the north. There are several boats out, but their crews soon retreat homeward before the approaching storm. It comes on rapidly, with vivid lightning striking the northern earth and heavy thunder following. 

Just before, and in the shadow of, the cloud, I see, advancing majestically with wide circles over the meadowy flood, a fish hawk and, apparently, a black eagle (maybe a young white-head). The first, with slender curved wings and silvery breast, four or five hundred feet high, watching the water while he circles slowly southwesterly. What a vision that could detect a fish at that distance! The latter, with broad black wings and broad tail, hovers only about one hundred feet high; evidently a different species, and what else but an eagle? 

They soon disappear southwest, cutting off a bend. The thunder-shower passed off to the southeast.

H. D. Thoreau, Journal, August 25, 1856

Why is the black willow so strictly confined to the bank of the river? See August 7, 1858 ("The most luxuriant groves of black willow are on the inside curves, —but rarely ever against a firm bank or hillside, the positive male shore. “); August 15, 1858 (“I notice the black willows . . . to see where they grow, distinguishing ten places. In seven instances they are on the concave or female side distinctly.”); August 19, 1858 (“I noticed the localities of black willows as far up as the mouth of the river in Fair Haven Pond, but not so carefully as elsewhere, and from the last observations I infer that the willow grows especially and almost exclusively in places where the drift is most likely to lodge, as on capes and points and concave sides of the river, though I noticed a few exceptions to my rule.”)