Page:The Journal of geology (IA journalofgeology11893univers).pdf/64
peared clear, and we could only trace one part of solid matter held in suspension by 25,925 of water. Nothing could more forcibly illustrate the formation of deltas. The river retains matter held in suspension by its water within its ordinary channel as long as its velocity is maintained; as soon as it enters a lake or an estuary checking regular currents, the matter held in suspension is dropped."
That is to say, in flowing 142 miles in its navigable channel and through its delta the river dropped about 15 per cent. of the load which it bore at Salto; and beyond the delta in still water it dropped 48 per cent. more; leaving it but 37 per cent. of the original load to be carried past Higueritas to the estuary of the La Plata. Or stating the proportions in terms of the sediment brought through the delta to the head of the lake, 57 per cent. was deposited and 43 per cent. escaped. It would be desirable to determine in what ratio the deposit is made in the upper and lower reaches of the lake, but Revy gives no data between Fray Bentos and Higueritas. He states however that the lake is without islands, although it is shallow with the exception of a deep channel half a mile wide; but just above Fray Bentos islands indicate the present front of the delta. The occurrence of these advance elements of the delta only in a limited distance indicates that the bulk of deposition is on the delta's front, and that the sediment which passes beyond is that which the slower current of the lake can hold in suspension.
The deposits of the extinct lakes Bonneville and Lahontan have been fully described by Gilbert and Russell, but the lake beds of the west still present rich fields for study of deposition under simple conditions in fresh and salt water.
(c) Alterations of Current.—When a land-locked water body is open to the ocean it is subject to influx and reflux of tides, but the rivers pouring into it may possess volume sufficient to exclude salt water; it is then a fresh-water estuary, which receives the sediments as well as the waters of its tributaries. The currents in such an estuary are periodic, changing with the flood and ebb, and the conditions of deposition vary accordingly. The