[Sidenote: Currents at the Mouth of the Dee and other Rivers.]
In many points, it is found, that the operation of the tidal waters of extensive arms of the sea, bears a close resemblance to what is observable upon the small scale in the currents of rivers, especially at their junction with the ocean. An interesting example of this occurs on the river Dee, which falls rapidly into the harbour of Aberdeen. Here the author having occasion to make some observations on the tides in the summer of 1812, stationed several assistants at low-water mark, between the entrance of the harbour and the bridge, about two miles up the river. The waters of the Dee, even at the entrance of the harbour, have almost a constant current seaward, notwithstanding the opposite direction of the flood-tide of the Ocean. On the occasion alluded to, one of his assistants, a very intelligent shipmaster, continued at his post while the water flowed up to his middle; and, when accosted about his situation, he significantly observed, “That it was rather extraordinary, as the stream had never ceased to indicate the continuance of an ebb-tide, while the water was still rising upon his body.”
[Sidenote: Water salt at bottom and fresh at top.]
In connection with these observations on the tides, some experiments were also made with an instrument adapted for lifting water from considerable depths, without the possibility of its intermingling with the surface water. By means of this instrument, the water at the bottom of the Dee, at Aberdeen, was found to be salt, while that at the surface was quite fresh. These streams of fresh and salt water run in distinct currents, and in contrary directions, the salt water, from its greater specific gravity, flowing at the bottom of the river, and fluctuating with the level of the ocean, while the fresh water is actually lifted upwards, and continues all the while to flow seaward on the surface of the salt water. Towards the point of high-water, however, the flood-tide gains strength on the margin of the basin of the harbour, where the water becomes salt, and forms an eddy-tide in a contrary direction to the central stream, which is observed still to run toward the sea. Having made similar observations on the waters of the Thames, the Garonne, and other rivers, with nearly the same results, after making allowance for the more level state of the country, in the track of these great streams,--it is concluded, that the currents at the embouchure of rivers bear a strong resemblance to the operation of the in and off shore tides of the ocean.