Some Creeks Highly Susceptible to Road Runoff
Most of the sediment from forest harvesting that ends up in creeks used for domestic water in British Columbia's Kootenay region originates from logging roads. Cutblocks, by comparison, contribute negligible amounts. But from a given length of road the amount of sediment flowing into a creek varies considerably among watersheds.
This is the case for Redfish Creek near Nelson compared with Gold Creek near Cranbrook, two watersheds without lakes where sediment reaching creeks mostly travels in runoff from spring snowmelt. A significant quantity of sediment suspended in the water at Redfish is attributable to logging roads. Sediment loads eventually decline when road cuts revegetate and when traffic diminishes after logging is completed. Gold Creek is much less sensitive to development, with roads having an inconsequential impact on sediment yield.
Differences in precipitation or topography don't explain the variation in suspended sediment yields between the two watersheds. Instead, permeability of the underlying rock plays a key role in how sensitive these watershed are to road building.
The Gold drainage is formed of highly-fractured sedimentary rock. Runoff seeps into the bedrock and flows into the creek as groundwater. Redfish, in contrast, is underlain with solid granite. The geology more directly connects roads with streams, causing water to percolate out of road cuts at the rock-soil interface. Water enters into Redfish Creek as surface or sub-surface water, carrying substantial quantities of sediment.
Peter Jordan. 2006. The use of sediment budget concepts to assess the impact on watersheds of forestry operations in the southern interior of British Columbia. Geomorphology. 79(1-2): 27-44.