636 THE FRESH-WATER LOCHS OF SCOTLAND 
increased to a maximum of feet. The lower degree of trans- 
parency earlier in the season was due to the fact that the snow on the 
mountains was melting more or less rapidly, and the streams in 
consequence were swollen and more or less turbid. As summer 
advanced the streams became smaller and their waters were clear. 
The maximum transparency of these lakes exceeds by 10 feet that 
found in the lakes of South-Eastern Wisconsin in 1900, and by 21 
feet that found in Winona Lake, Indiana, in 1901. 
The Red River, one of the tributaries of the Mississippi in its 
lower course, furnishes examples of two different kinds of lakes. The 
head-water streams bring down more detritus than the trunk stream 
can carry away, and its flood-plain is built up so fast that the smaller 
tributaries cannot fill up their valleys at the same pace ; they are 
consequently ponded, and many lateral lakes are formed, arranged, 
as Davis aptly says, like the leaves on a twig. Lakes are also formed 
on this river by the blocking of streams by timber-rafts, compar- 
able to the "sudds"" of the Nile. The rafts form floating islands, 
and dam the streams so as to cause their waters to spread out in 
shallow lakes 20 to 30 miles in length, sometimes many square miles 
in area, and covered with living vegetation.^ 
The Mississippi in its lower course is an old river with a very 
gentle slope meandering in broad curves through a wide flood-plain. 
The loops are frequently cut off, and crescent-shaped or " ox-bow " 
lakes are left. At its mouth the river is rapidly building a low- 
grade delta (with lakes) out into the Gulf of Mexico. 
Lake Pontchartrain, the largest delta basin at the present time, 
40 miles long by 25 miles broad, and with a depth of only 27 feet, 
lies between the Mississippi and the Pearly River. It communicates 
with Lake Maurepas on the west, and with Lake Borgne on the east. 
Lake Borgne, in the same region, is of later origin and is not yet 
completed. It communicates with the Gulf of Mexico in the east, 
and is connected with Lake Pontchartrain on the west by the Rigolets 
Pass, about 10 miles long. 
Eiver A great extent of country drained by the Columbia River and its 
Columbia. l-jjg Snake River, in Idaho, Oregon, and Washington, is 
built up of vast lava-sheets which have converted a broad depression 
between the Rockv and the Cascade Mountains into an extensive 
plateau, the great plain of Columbia. These lava-sheets formed great 
dams across the valleys, and the marshes round the edges of the 
Snake River lava-sheets seem to be lakes formerly retained by lava 
barriers but now verging on extinction. The Columbia now skirts the 
1 Chas. Lyell, Principles of Geology, 11th ed., vol. i. p. 441 ; Humphreys and 
Abbott, op. cit., p. 37. 
