source of sediment within the proposed sale area. Based on recent surveys it is estimated that 

 approximately 23 tons of sediment/year is eroded from this gully and delivered directly into Bear 

 Creek. Other potential sources of impact to water quality which are not addressed under the no 

 action alternative are the poor condition of many of the existing roads within the proposed sale 

 area and the presence of several unimproved stream crossings (fords). 



Cumulative Watershed Effects 



Cumulative watershed effects are off-site, hydrologic effects of the proposed actions when 

 combined with past or other ongoing activities. Hydrologic effects include downstream 

 changes in hydrology, channel stability, and sediment production, transport and storage. 

 Existing cumulative watershed impacts due to increased water yield, flow alteration and 

 channel instability appear to be largely attributed to the lack of flow regulation for the 

 existing irrigation ditch diversion, severe irrigation ditch erosion and localized areas of 

 concentrated livestock grazing. 



A riparian grazing enclosure was constructed on the DNRC ownership in lower Bear Creek 

 in 1996 to address stream bank instability caused or exasperated by concentrated livestock 

 use. Stream bank stability and riparian vegetative cover within the enclosure have improved 

 as expected. Additional recovery is expected as a riparian shrub community re-establishes. 



Other cumulative watershed impacts are due to segments of existing road with direct 

 sediment delivery to streams and ephemeral draw bottoms. Existing sediment sources were 

 identified and inventoried during detailed sediment sources surveys completed by a DNRC 

 hydrologist, a soil scientist and a private contractor. 



The ECA method was used to predict the cumulative increases in average annual water yield 

 for each watershed analysis area under each of the proposed action altematives. This 

 information was used to evaluate the potential for detrimental channel impacts due to 

 increased intensity and duration of peak flows. The results of water yield modeling are 

 summarized in the table listed below: 



79 



