Sediment production in a coastal watershed: legacy, land use, recovery, and rehabilitation
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Sediment production has been measured for nearly half a century at the Caspar Creek Experimental Watersheds. Examination of this sediment record provides insights into the relative magnitudes and durations of sediment production from management practices including road construction, selection harvest and tractor skidding, and later road decommissioning. The 424-ha South Fork was harvested under standards that applied before passage of the 1973 Forest Practice Act. Regression analysis of annual suspended sediment loads on peak flows indicates that sediment production roughly doubled, with a return to pretreatment levels about 11 years after harvest ended. However, sediment production again increased in the 1990s as road crossings deteriorated in response to large storms. Road crossings decommissioned in 1998 eroded a volume equivalent to more than half of the total yield in 1999 and enlarged another 20 percent over the last decade. Suspended sediment yields since decommissioning were reduced only for small storms. Recent assessment of 1970’s era roads and skid trails found 443 remaining stream and swale crossings. Stream crossing have eroded an average volume of 10 m3. Stream diversions are common, and many sites have the potential for future diversion. Diversions along incised roads and skid trails contribute to episodic sediment inputs. Mainstem sediment loads are elevated relative to those at tributary gages located above the decommissioned riparian haul road, indicating that sediment yields at the weir are enhanced along the mainstem itself. Since turbidity monitoring began in 1996, South Fork mainstem turbidities have exceeded ecosystem thresholds of concern a higher percentage of time than those in the North Fork.