Cowichan River – Stoltz Bluff Sediment Remediation

Stoltz Bluff is a deposit of glacial sediment that extends for approximately 600 m at the outside of a natural meander bend on the Cowichan River, 27 km upstream of Cowichan Bay. Stoltz Bluff was previously identified as the largest point source of fine sediment on the river, representing on average 35-45% of the river’s annual total suspended sediment (TSS) load (KWL Assoc. Ltd. 2005). This is significant in that effective sediment management was identified as the second highest fish habitat restoration priority in the Cowichan Recovery Plan, prepared for Cowichan Tribes’ Treaty Office (LGL Ltd. 2005).
The primary objective of a potential Stoltz Bluff remediation project evolved over 25 years to become “the effective control of massive sediment transport from the bluff (i.e., 10,000-28,000 m3/year since 1993) (KWL Associates Ltd. 2005)), that had been negatively affecting Cowichan River fish habitats and stocks for many kilometres downstream (Burt and Ellis 2006).” The recent Cowichan River Watershed Health and Chinook Initiative (Ayers 2017) reemphasized the value of improved year-round water quality and salmonid spawning habitat as an ongoing strategic management objective.

Beginning in July 2006, the BC Conservation Foundation (BCCF) coordinated a major sediment remediation project at Stoltz Bluff. Work included construction of an engineered 600 m rip-rap berm and terrace, complete with a series of weirs, channel gradient controls and bioengineering treatments designed to move river flows away from the base of the bluff and prevent further bank erosion and major slope failures.

The initial phase of the project took 10 weeks to build, and was supported by a group of seven partners who contributed $830K in funds and in-kind construction materials (including $250K from PSC). For 10 years following the project’s inception, there was a significant improvement in the river’s water quality (i.e., TSS and turbidity) and downstream fish habitats (Gaboury et al. 2012), which is thought to have contributed to incremental gains in annual returns of the river’s fall Chinook and chum salmon stocks, as well as winter steelhead.

However, since 2014, there has been a notable (and even dramatic) change in slope stability at Stoltz Bluff, apparently prompted by the cyclical return of wetter winter weather conditions (McQuarrie 2017). Mass wasting events in the Bluff’s gullies and numerous smaller slope failures have collectively overwhelmed existing sediment retention infrastructure, resulting in higher sediment loads entering the Cowichan River. If this is not effectively mitigated in subsequent annual maintenance activities, a decade of improved spawning conditions will likely become incrementally reversed.