10 Smithsonian Exploration in Alaska in 1904 



The ascent of the Porcupine was commenced June 23, progress 

 being made by " tracking " and " poHng." The lower one hun- 

 dred and twenty-five miles of the Porcupine, from Fort Yukon to 

 the Lower Ramparts, flows by a tortuous course through the low 

 forested region known as the Yukon Flats. Its course forms a 

 series of curves of one to three miles' radius alternately sweeping 

 from right to left, the channel being entirely confined by banks of 

 unconsolidated alluvium nowhere exposing rocks older than Pleisto- 

 cene age. The banks are of an average height of about twenty 

 feet above the normal level of the river, but are seldom sufficiently 

 elevated to prevent their overflow by the spring floods. The dif- 

 ference of level between the mouth of the Porcupine and a point 

 one hundred and twenty-five miles above produces a gradient over 

 which a torrential current is only prevented by the extremely tor- 

 tuous course followed by the river over its flood plain. The cur- 

 rent averages about three miles an hour through this part. It 

 presents the typical features of meandering erosion, cutting away 

 the banks on the concave side and depositing the material removed 

 lower down on the opposite convex side or bars. These bars are 

 quite shallow in most cases and being frequently strewn with stumps 

 and stranded trees necessitate continuous wading on the part of the 

 " trackers " with the attendant annoyance of the tow-line frequently 

 becoming fouled with entangling branches, roots, etc. These fea- 

 tures present a marked contrast in the character of the banks. Those 

 of the outer curves are precipitous, owing to the undermining and 

 consequent crumbling of the banks. Being frequently covered with 

 a thick growth of spruce the river cuts a path through the forest 

 leaving the trees standing as grain does beside a clean cut swath, 

 frequently the sections give exposures of solidly frozen peaty layers 

 and also the edges of lenticular beds of clear flood-plain ice. The 

 shimmering silence of the nightless days of summer in these arctic 

 solitudes is often harshly broken by crashing splashes along these 

 undercut banks where massive blocks of frozen earth topple over 

 with their incumbent trees to disappear with muddy gurgles beneath 

 the silt-laden current. In plate i, fig. i, is shown a cut bank in 

 Yukon Flats, with timber and overhanging mantle of tenacious 

 turf characteristic of all river banks of Alaska where unconsoli- 

 dated deposits are undermined by the current. 



On the inside of the curves are the low, gently sloping banks of 

 recent flood-plain deposits known as bars. In typical sequence these 

 are current-bedded gravels succeeded at a higher level by sand 

 beds which in turn pass beneath deposits of fine silts. The gravels 



