Stream dynamics and ecology of the Assabet

July 16 and 18. 

Afternoons, I sounded the Assabet as far up as the stone bridge. 

This bridge, as I see by the town records, was talked about (i. e. the building) in 1807, and was probably built that year or the next (though E. Wood says that the Turnpike Company, who then proposed to build it, did not fulfill their contract). Shattuck's date, 1802, is wrong. 

Accordingly, by building this narrow bridge here, twenty-five feet in width, or contracting the stream to about one fourth its average width, the current has been so increased as to wash away about a quarter of an acre of land which rises a dozen or fourteen feet above water (or at least an acre four feet in depth) and dig a hole six times the average depth of the stream, twenty-two and a half feet deep, or considerable, i. e. three feet, deeper than any place in the main stream from Sudbury Causeway to Atkins's boat-house bend, and all this in fifty years.

Yet the depth under the bridge is only two and a half feet plus. It falls in four rods from two and a half to twenty-two and a half. A considerable island has been formed there, at least three feet and a half above low water, composed of sand, and, two or three rods lower, are deposited the stones, generally larger than a hen's egg, without sand, forming bars and islands quite distinct from the former. 

This is much the swiftest place on the stream thus far and deeper than any for twenty-five miles of [the] other stream, and consequently there is a great eddy, where I see cakes of ice go round and round in the spring, and, as usual, the shoal water and islands formed by the ruins of the bank and of the bottom are close by. As usual, the shoal water is produced by the rapidity of the current close by.

The sand and gravel are deposited chiefly in the immediate neighborhood of the swiftest water, the swift water producing an eddy. Hence, apparently, the sandy islands at the junction of the rivers, the sand-bar at the swift place on the Assabet, etc.

Contract the stream and make it swift, and you will wear a deep hole and make sand-bars and islands below. 

The stream is remarkably different from the other. It is not half so deep. It is considerably more rapid. The bottom is not muddy but sandy and occasionally stony. Though far shallower, it is less weedy than the other. In the above distance weeds do not anywhere grow quite across it. A shallowness of two and a half feet does not necessarily bring in weeds, and for long distances three feet is clear of weeds. This is owing, perhaps, not only to the greater swiftness of the current, but to the want of mud under the sand. 

The banks and bars are peculiar. They are commonly composed of a fine sand mixed with sawdust, shavings, etc., in which the black willow loves to grow. I know of no such banks on the main stream. 

Again there are comparatively few of the large floating potamogetons here. (I do not remember any of the very largest species.) The weeds are chiefly bur-reeds and a slenderer potamogeton and an immersed species (I speak of weeds in the middle). 

You wonder what makes the difference between this stream and the other. It seems impossible that it should be a geological difference in the beds of the streams so near together. Is it not owing simply to the greater swiftness of this stream? 

Does not this produce a sandy and gravelly and stony bottom, and so invite a different fauna and flora? I suspect that a fall of two or three inches more in a mile will produce a different fauna and flora to some extent, — the fresh-water sponge, the wood tortoise, the sucker, the kingfisher, the stone-heaps. 

It is remarkable how the stones are separated from the sand at the Eddy Bridge and deposited in a bar or islands by themselves a few rods lower down. The sand bar there, partly under water, looks exactly like a snow drift. It is a narrow, sharp ridge, extending southwest from the island, with deep water on each side. The sand carried round by the eddy falls there where the ice is observed to loiter most. The large stones are perhaps swept away by a stronger current beneath.

The bars and banks of this stream are peculiar, i. e. of fine sand without mud. This indicates a fall and swifter water, and consequently it is on such a stream the mills are built and sawdust and shavings are mixed with such sand to form the bank. 

One such bank at the swift place has been recently raised four or five feet above the present level by freshets. It is apparently advancing down-stream.  What is deposited by the eddy occasioned by the narrows is building it up, and so the stream is being narrowed further down. 

Eddies are the great builders of sand-bars and islands and banks. Any agent that stops the progress of the water downward builds up the bottom in some place. At the bottom of the deep hole at Eddy Bridge, I felt several water-logged trunks of trees and saw some, which probably were carried round and round by the eddy until they became water-logged and sank.

H. D. Thoreau, Journal, July 16, 1859