RECORDS MADE BY RIVER ICE. 117 



its mouth, and in the spring opens first at its head. Near its mouth it is 

 closed each year about the middle of October, but has been known to remain 

 open as late as the first of December. As winter approaches, ice forms along 

 its sides, leaving open water in mid-channel or where the current is swiftest. 

 The fringe of ice first formed is smooth, and can be easily traversed. As 

 the river falls, however, during the winter, it becomes much broken, and in 

 many instances quite impassable. When the cold is sufficiently intense to 

 completely close the river mouth, the swift current packs the new slush-ice, 

 and cakes broken from the sides, against this ice bridge. This process con- 

 tinues progressively up-stream till the river is completely ice-covered from 

 mouth to source. The freezing of the lakes on the upper waters of the 

 Yukon, I have been informed, is frequently delayed until December. 



From the manner in which the swifter portions of the river become ice- 

 covered, as well as from the breaking and subsidence of the ice due to the 

 shrinking of the river in very cold weather, the frozen river is almost always 

 rough and difficult to travel over.* 



The thickness of the ice on the lower river is stated by several residents 

 to be generally from ten to fifteen feet. Some of the tributaries of the 

 Yukon, which are veritable rivers in summer, are frozen solid to the bottom 

 during winter. In Forty-mile creek placer mining is carried on in winter 

 by cutting away the ice and thawing out the frozen gravel beneath by means 

 of large fires. The auriferous gravel is removed to the bank of the stream 

 and washed when warm weather returns. 



Spring Freshets. — In spring the river thawing first at its head frequently 

 initiates floods and ice gorges of great magnitude. At times the water behind 

 an ice dam rises thirty or forty feet, and if the bank of the river chances to 

 be low, inundates large areas. During these freshets immense quantities of 

 ice are borne along by the swift current and lodged in heaps on the river 

 banks. The annual movement of such large quantities of ice is accompanied 

 by results of geological interest. 



Rock Surfaces polished and scratched by River Ice.— The banks of the Yukon 

 where they are precipitous are frequently smoothed and polished in the space 

 between high and low water. The surfaces best showing these characteristics 

 are on the up-stream side of bold promontories. In such localities the smooth 

 surfaces are not infrequently scratched in an irregular manner. The scratches 

 are rudely parallel to the direction of the river current, but are not deeply 

 engraved. On the down-stream side of projecting rocks and cliffs the sur- 

 faces are rough and without striatious. These records are clearly due in la rge 

 part to the friction of ice descending the river. The scratches are mad.' by 

 sand and pebbles frozen in the ice. 



* The behavior of northern rivers in winter has been described by A. C. Inderson, in Jour. 

 Geograph. Soc. London, Vol. 15, 1845, pp. 307-371. 



