A STUDY OF TWIN LAKES, COLORADO. 153 



AFFLUENTS. 



The principal affluent is Lake Creek, which rises in the Sawatch Range. Some dis- 

 tance above the lakes the creek divides into two branches. The north or main branch 

 has its beginnings in amphitheaters up near the crest of the mountains, a little to the 

 northwest of Blount Elbert. It Hows south and southeast for a distance of about 10 

 miles (16 kilometers), and then east about 8 miles (12.8 kilometers) into Upper Lake. 

 The south fork rises in amphitheaters lying west and southwest of La Plata, flows 

 northeast about 8 miles, and joins the north fork. Powell (18!tl) states that Lake 

 Creek drains about 103 square miles (261 square kilometers) of high mountain coun- 

 try. At present, however, not all the water of the creek reaches the lakes. About 

 5 miles (8 kilometers) above Upper Lake considerable water is diverted into a ditch 

 which furnishes a water supply for placer mining in the vicinity of Granite. During 

 the late summer, when the water in Lake Creek is especially low, it is said that so 

 much is removed by this ditch tiiat frequently portions of the creek below are entirely 

 dry and large numbers of brook trout perish there. At the point where Lake Creek 

 emerges from its canyon there was formerly a fall of sufficient height to prevent the 

 trout from ascending the stream, but a few years ago much of this rock was removed 

 b}- blasting and trout may now ascend the creek without difficult}'. 



About a dozen other streams of various size* contribute their quota of water to 

 the lakes. 



CHAKACTEi; OF SHORES AND BOTTOM. 



As stated before, the lakes are entirely surrounded by morainal detritus, so that 

 the shores are composed very largely of sand and gravel. Li places, however, there 

 are rocks varying in size from mere cobblestones to huge bowlders. At present the 

 cutting action of the waves on the shore is very slight in most places. At two points 

 on Lower Lake, however, one on the north side and the other on the south side, the 

 waves have recently been cutting the shores very rapidly. The increased height of 

 the water caused by the dam has directed wave action at these points against loose 

 morainal banks that are steep and easily cut away. Along some parts of Upper 

 Lake the action of the ice on the shores was well illustrated by the small ridges of 

 shore material that had been pushed up just a few feet back of the water's edge. 



The bottom of the shallower parts of the lakes is sandy and gravelly for the 

 most part, but it is composed of bowlders of various sizes in some places. In the 

 deeper water a marly deposit covers the liottom. 



TRANSPARENCY OF THE WATER. 



The transparency of the water of both lakes varied somewhat during the period 

 of these ol)servations. It was found that, in general, a Secchi's disk just disappeared 

 from view at a depth of about 18 feet (5.5 meters) early in July, and the water 

 gradually became more transparent as the season advanced, so that, by the middle of 

 August, this depth had increased to a maximum of 29.5 feet (1» meters). The low 

 transparency early in the season was due to the fact that the snow on the mountains 

 was melting rapidly and the streams in consequence were swollen and more or less 

 roily. As summer advanced they became smaller and their water became clear. 



