40 DYNAMICAL GEOLOGY. 



The backing up of the river-water by the tides causes it 

 to overflow. There is, therefore, here a more or less ex- 

 tensive marshy or swampy flood-plain. 2. The river here 

 not only forms a bar, but also a more or less extensive 

 flood-plain deposit. 3. The river winds tortuously and 

 in many channels through the soft, marshy soil, forming 

 many marshy isles. These facts are shown in Fig. 17. 

 In fact, we have here many of the phenomena of a river- 

 delta. The Hudson Eiver, for example, is an estuary, one 

 hundred and twenty miles long. The tide runs up and 

 meets the river-current, and makes still water about 

 twenty miles below Albany. At this point is the bar. 

 At this point also is an extensive marshy overflow-land, 

 through which the river winds its tortuous course. The 

 same phenomena are seen at the head of the Bay of 

 San Francisco. The river here winds, by many tortuous 

 channels with islands between, through an extensive 

 marsh (fo^e-lands). The bar is also, of course, found 

 here. 



Removal of Bars. If a bar be scraped away, it will 

 be re-formed by the same agencies which originally formed 

 it. Only constant dredging can improve it. If the river- 

 channel be contracted by dikes so as to increase the ve- 

 locity of the current, it will indeed scour out the bar, but 

 the latter will again form at a new point of equilibrium a 

 little lower down. In rivers forming deltas the bar has 

 been successfully removed, in some cases, by means of 

 jetties extending beyond the mouth of the river into the 

 sea or gulf. The now swifter current scours out the bar, 

 and the sediment is delivered in deep water, where it 

 must deposit a long time before the bar is re-formed. 

 The most remarkable examples of such improvement of 

 bars are at the mouth of the Danube by the Austrian 

 Government and at the mouth of the Mississippi by the 

 United States Government. 



We have now traced the agency of rain jind rivers from 



