periods. The minimal areal extent- of exposed gravel bars also generally 

 leads to extensive damage to the river system either by use of extended 

 lengths of river channel or by disturbing vegetated floodplains. 



Glacial origin streams are not common in the area of study; only three 

 sites situated on this type of river were studied. These were on Phelan 

 Creek and the Tanana River. Because these systems are of mountain origin, 

 the availability of weathered parent materials is not limiting and usually 

 large quantities are available. The Phelan Creek site was situated near the 

 glacier and gravel was abundant across a wide area. The proximity of the 

 site to the glacier strongly influenced the seasonal fluctuations in dis- 

 charge. During winter, water flow from the glacier is greatly reduced and is 

 supplemented by that from associated springs. This reduced flow exposes vast 

 expanses of gravel for extraction. 



The Tanana River sites are well downstream from the river origin, there- 

 fore, water flows throughout the year because of the numerous spring- and 

 groundwater-f ed tributaries entering the river. Affects include those associ- 

 ated with braided channels that flow in winter. In these systems, however, 

 ice cover on channels is more of a factor than on a system like Phelan 

 Creek, near its origin. 



The availability of gravels in glacial origin rivers makes them a 

 viable source of materials even when needed in large quantities. This is 

 basically true for systems of all sizes although on smaller rivers the 

 localized deposits are more restricted. 



Most rivers in northern and interior Alaska are of mountain or foothill 

 origin. The weathered parent material in the headwaters provides large 

 quantities of gravels, particularly in the mountain systems. These rivers 

 are fed by springs, melt water, and runoff and, therefore, discharge fluc- 

 tuates seasonally. Spring-fed systems can be expected to have at least 

 Intergravel flow in winter. Moderate to steep channel slopes are normal 

 in the headwaters but these slopes are influenced by the length of the river 

 and the topography through which it flows. Bed load movements are usually 



552 



