Aufeis is defined as areas of ice which have developed by a sequence of 

 events of overflowing water on top of the previous ice surface. The general 

 mechanism for the growth of aufeis involves an increase in the hydrostatic 

 pressure due to a reduced flow area; when the pressure exceeds the elevation 

 of the ice surface, overflow onto this surface results and subsequently 

 freezes. The overflow causes the pressure to decrease and ice surface ele- 

 vation to increase. This sequence continues to repeat until the source water 

 cannot produce sufficient pressure to exceed the elevation of the ice sur- 

 face. Three requirements for the formation of aufeis are given by Carey 

 (1973); (I) significant ground water or under-ice flow, (2) growth of ice to 

 the channel bed or near the bed, and (3) subsurface constriction such as 

 bedrock, less pervious soil, or permafrost. 



Changes Due to Gravel Removal 



An organized program of winter and spring observations of aufeis and 

 breakup were not included in this study. Therefore, much of the following 

 discussion is based on observations of auf e is and ice jamming potential, 

 rather than of actual aufeis and ice jams. However, at two sites, Washington 

 Creek (Figure 38) and Oregon Creek, large areas of aufe is were observed in 

 early June. Incidental winter observations at a few other sites documented 

 the existence of aufeis. 



Ice jams could be caused by several aspects of floodplain gravel re- 

 moval. In rivers which are increased in width and depth by the gravel re- 

 moval, such as by in-channel mining, the velocity would decrease causing the 

 ice floes to gather. At the downstream end of the gravel removal area these 

 floes could jam where the channels constrict back to the natural width. This 

 ice jam could cause flooding in and upstream from the gravel removal area 

 and possible bed scour beneath the ice jam. River channels which are widened 

 causing shallower depths, such as by removing bars adjacent to the channel, 

 could cause ice jamming by grounding the ice floes. Another potential mechan- 

 ism for ice jam formation resulting from a gravel removal operation is the 

 blocking of ice floes by flow obstructions in the form of overburden piles, 

 stockpiles, or dikes. 



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