BARS. 33 



(Fig. 24, d d) the bar is formed by the contact of the river-current 

 with the still water of the ocean. It is most marked in the case of 

 estuaries. The outflowing tide scours out the estuary, carrying with it 

 sediment partly brought down by the river, and partly the debris of 

 land eroded by the inflowing tide. The larger portion of this is dropped 

 as soon as the tidal current comes in contact with the open sea and is 

 checked by it. They are usually irregularly crescentic in form. Such 

 are the bars at the mouths of all harbors. In the second position they 

 are found just where the upward current of the inflowing tide meets 

 the downward current of the river, ajad makes still water. At this 

 point we have not only a bar, but usually also an extensive marsh 

 caused by the daily overflow of the "river. Through this marsh the 

 river winds in a very devious course, as is common in all rivers whose 

 banks are alluvial. 



Thus, then, in rivers like the Mississippi, emptying into tideless seas 

 and forming deltas, there is but one bar, viz., that at the mouth ; while 

 in rivers forming estuaries there are two bars, an outer and an inner. 

 This inner bar may be many miles up the river. In the Hudson River 

 the inner bar is a hundred and forty miles up the river, and only a few 

 miles below Albany. This is really the head of tide- water in this river.* 



Bars, being produced by natural and constantly-acting causes, can 

 not usually be permanently removed, though they may be sometimes 

 greatly improved. If they are scraped away by dredging-machines, 

 they are speedily reformed on the same spot. If we cause the river 

 itself to remove them, as has sometimes been done by narrowing the 

 channel and thus increasing the erosive power, we indeed remove the 

 bar, but it is reformed farther down-stream at a new point of equi- 

 librium. In some cases, however, bars have been permanently removed. 

 This has been done for the Danube, and recently by Capt. Eads for the 

 Mississippi, by the construction of jetties running out to deep water. 

 These confine the current, increase its velocity, and cause the river to 

 scour away its bar, and thenceforth to deposit its sediment in water so 

 deep that it will require centuries to build up again from the bottom, 

 and re-form the bar. 



We have thus traced river agencies from their source to the sea. 

 This brings us naturally to ocean agencies. 



* There is another important principle affecting the formation of bars in rivers empty- 

 ing into seas, viz., the flocculation and consequent precipitation of clay sediments, by salt- 

 water (Hilgard). 



