160.Accidents in River Development. -While a river is devel-oping its drainage area many accidents may happen to it. The competi- tion of other rivers in the same region affect it. The river that has theshortest course to the sea or the most easily eroded bed has the ad- vantage. It lowers its valley more rapidly, thus giving its side streams steeper grades and enabling them to wear back faster into the upland, and thus to gain more than their share of the drainage of the region.
Fig. 113.Fig. 114.
As soon as the unappropriated drainage area has been channeled, it begins to push back the divides of its neighbors, thus appropriating some of the run-off they may have acquired. In Figure 113 a case of this kind is shown. The river A reaches the sea by a long course, while the streams B, C, D have short courses. These short streams have steeper grades than A and thus are able to gnaw back and cut down their valleys faster. Thus they push the divide EF farther and farther toward A.
In Figure 114 a stage is shown in which the divide has been pushed back toward A and at one point has approached very near to the upper part of the branch G. In Figure 115 it has been pushed across this branch and the stream B has tapped G and appropriated its headwaters. This isa case of what is called beheading
or piracy.
Fig. 115.
As A has lost some of its water it erodes its valley even more slowly than before, and a branch of the stream D may take away the headwatersof its branch H. If time enough is allowed, a branch of the stream C may completely behead A, leaving only its lower trunk as an independent river. Cases of river piracy are most interesting phenomena.
A river may by some accident have its supply of sediment greatly increased, causing it for a time to build up its valley floor instead of eroding it, thus forming a filled river valley. When the supply of sediment fails, the river begins cutting down the filled valley, leaving terraces along the sides to mark the successive levels at which it flowed.
River terraces are often very prominent along our northern rivers, since by the melting of the ice at the close of the last Glacial Period these rivers were supplied with a vast amount of sediment which they were unable to carry away and so deposited on their valley floors. When the supply ceased, they eroded their valleys, leaving terraces along the sides.
The region in which a river is situated may be elevated, thus affectingits normal development and beginning a new cycle in its history. The elevating may take place over its whole drainage area or only over a part of it. If the whole region is elevated, the energy of the entire river is revived, and the river may be called a revived river.
RIVER TERRACES.
The river is now cutting down its former plain, leaving terraces.
INCISED MEANDERS.
The elevation may take place at any time during the history of the river. If it takes place after the river has become old and is meandering on its flood plain, the river willbegin afresh to cut down its valley. But as its meandering course has been established, the trench that it now cuts is not like that of a young river, but is a meandering trench, and what are called intrenched meandersare formed. This region will
INTRENCHED MEANDERS.
have the steep V-shaped valleys characteristic of a young region and the well-developed drainage and meandering rivers characteristic of a mature region. The Palmyra, Virginia, sheet shows these characteristics. The main rivers meander in steep valleys. A profile shows these valleys to be steep and V-shaped with broad, rounded uplands between, well provided with drainage channels.
When the elevation extends beyond the mouth of the river, the river must prolong its course over the emerged land in order to reach the sea. It may happen that rivers which formerly entered the sea at different points, in extending their courses over the emerging continental shelf, join some large stream and all enter the sea through it.
This is what happened to the rivers now forming branches of the lower Mississippi when the coastal plain bordering the Gulf of Mexico was elevated. These formerly entered the extended Gulf by separate mouths, but when the land rose and forced the water of the sea back, their extended courses joined them to the great central river, thus vastly increasing its drainage area and the volume of water it poured through its mouth into the sea. Many of the great river systems of the world have been built in this way. These may be called engrafted rivers.
In Figure 116 the rivers all enter the sea at the old coast line GHby separate mouths. When this region is elevated so that the coast is at IK, the rivers E, D, B find that their easiest course to the sea is byFig. 116.
engrafting themselves upon the river C, and thus they all four find their outlet at one point L. The rivers F and A still maintain their independent courses.
It may be that the elevation takes place over only a part of the river"s course. Then the river is dammed back and laked on the landward side of the elevation and obliged to seek a new course for itself, thusbecoming a reversed river, or else it is strong enough to cut its beddown as fast as the land rises, and thus maintain its course. Such a river is called an antecedent river, as its course antecedes the uplift which naturally would have determined its course. The Columbia River has maintained its course through uplifts which have reached thousands of feet.
Not only may a river be elevated, but it may be depressed. In this case its rate of erosion is diminished, and the river becomes marshy where the grade is low. Where the river valleys approach the sea they will be submerged or drowned.
ALLUVIAL CONES.
Formed at the foot of each mountain gully.
The se d row ne d rive r
valleys form some of the finest harbors on the coast. San Francisco Bay, Narragansett Bay and New York harbor are examples of protected harbors due to the submergence of rivers. The mouth of the Hudson was formerly some seventy miles to the east of Long Island, that of the St.