TOPOGEAPHIC FEATUKES OF THE DESERT BASINS. 25 



THE CHEWAUCAN BASIN AND ABERT LAKE. 



The Chewaucan Basin lies between the north-south fault scarps of two outward 

 dipping monoclines. On the west is the Winter Eidge, bounded by the 2,500-foot 

 scarp west of Summer Lake and dipping westward to the valleys of the Klamath and 

 the Deschutes. On the east is the similar scarp east of Abert Lake, and beyond that 

 the gentle eastward slope of the monocline down to the Warner Valley. The two 

 fault lines which determine these scarps come together south of Abert Lake and are 

 lost in a region of general uplift which forms the divide between the Chewaucan Basin 

 and the Goose Lake Basin to the south. Toward the north the fault lines diverge, 

 and the basin is bordered by the rolling lava plateau already described in connection 

 with the Christmas Lake Valley. The drainage of this is scarcely at all determinate 

 and the divides between the Chewaucan and the Alkali and Christmas Lake basins 

 are correspondingly uncertain. 



The deepest depressions of the basin lie just beneath the greatest heights of the 

 scarps and are occupied respectively by Summer Lake on the west and Abert Lake 

 on the east. Both of these lakes are very shallow. Summer Lake is bordered by a 

 playa area on the north and east and Abert Lake on the north only. Summer Lake is 

 about 200 feet higher than Abert and was once connected with it through the Che- 

 waucan Marsh. This connection is now broken just south of Summer Lake by a low 

 alluvial dam probably due in part to delta and fan formation by the Chewaucan Kiver, 

 which enters the valley just at this point. The divides surrounding the basin as a 

 whole are high and structural and there is no indication of any previous overflow. 

 Abert Lake is surrounded by terraces indicating that the water body has been much 

 larger and deeper than now. (PI. Ill, fig. 1.) The heights of these terraces have not 

 been measured accurately, but hand-level and aneroid measurements place the highest 

 of them at about 200 feet above the lake. The terraces can be traced about the Che- 

 waucan Marsh, but not into Summer Lake, the present elevation of this lake being 

 very nearly that of the highest terrace. It is probable that the present Summer Lake 

 was once a shallow bay or filled estuary of the early water body, and that it was then 

 cut off from the main body by wave accumulation and delta building, the details of 

 which have been obscured by subsequent rain wash. It is impossible to determine 

 the date of this separation or whether or not Summer Lake continued for a while to 

 overflow into the Abert body. The present inclosed character of the lake may have 

 originated during the maximum of the lake expansion or it may have been initiated 

 only very much later. The writer inclines to the latter opinion, but, in any case, 

 Summer Lake was once a tributary to Abert, and any extensive salt accumulations 

 should be looked for in or under the latter rather than in the former. 



The drainage of both Summer and Abert Lakes is now slightly less than formerly, 

 because of the decay of the drainage from the pans of the plateau region to the north, 

 as already described in connection with the Christmas Lake Valley. The Antelope 

 Valley south of Abert Lake was apparently once a small mountain lake which has 

 been drained by the cutting of the gorge of Crooked Creek and at the same time filled 

 by alluvium. At present it has unrestricted though meager drainage to Abert Lake. 

 Except as water is used for irrigation, the Chewaucan River and Marsh drain freely 

 into Abert. The area now tributary to Abert, including everything except Summer 

 Lake, is about 930 sqxiare miles. The Summer Lake drainage totals about 560 square 

 miles, making nearly 1,500 square miles for the two. These figures are open to slight 

 uncertainty because of the indefinite character of the northern divide. The present 

 area of Summer Lake is 75 square miles and that of Abert Lake 60 square miles. 

 19750°— Bull. 54—14 4 



