the form of increases through the mined reach resulting from channel cutoffs 

 and local slope redistributions affecting the pool-riffle sequence. Flow 

 obstructions in the forms of material stockpiles, diversion dikes, and 

 overburden piles have the potential for causing local scour, ice jam forma- 

 tion, and si I tat i on. 



Hydraulic Geometry. Gravel removal operations caused changes in the 

 natural cross-sectional shape of the active channels of approximately half 

 of the rivers included in the study. The backwater analysis was not complete 

 enough at some sites to confirm the hydraulic geometry change evaluation. 

 A comparison of power equation exponents for cross sections in disturbed 

 and undisturbed areas (Table 6) indicated a varied response to gravel re- 

 moval. The coefficients in the power equations must also be considered 

 to understand the effects of gravel removal. For example, at the Middle 

 Fork Koyukuk River-Downstream site, the top width increased at a slower 

 rate within the gravel removal area than outside of it. However, the coef- 

 ficients in the power relations were greater for the disturbed than the 

 undisturbed cross sections indicating that the top widths were larger at 

 low flows in the disturbed areas than the undisturbed areas and were similar 

 in both locations at higher flows. A qualitative evaluation of this effect 

 can be made by comparing the relative channel widths in the material site 

 at low flow and flood flow (Channel width and Flooded area. Table 7). 



The coefficient in the power equation for the top width was greater 

 for the disturbed cross section than the undisturbed cross sections at 

 eight of the sites; this difference resulted from a consistently greater top 

 width at all discharges considered in the hydraulic analysis. The sites at 

 which this occurred were Gold Run Creek, Washington Creek, Nome River, 

 Aufeis Creek, Skeetercake Creek, Sagavan irktok River, and both sites on the 

 Middle Fork Koyukuk River. At Sinuk River the exponent of the power relation 

 for the top width was observed to be greater at the disturbed cross section 

 than at the undisturbed cross section. This difference indicates that the 

 gravel removal area had smaller top widths at low flows, but larger top 

 width at high flows, than the undisturbed cross section. 



108 



