Saving the Cowichan Estuary from drowning in a climate-fed ‘coastal squeeze’

High atop a dike hemming the Koksilah River as its fresh waters meet salt, red-winged blackbirds call out as they patrol their territory.

Noisy heralds of spring, the blackbirds return to the Cowichan Estuary each year to nest and protest human intrusion with sharp signature trills from the brush along the riverbank.

Today the interloper is Tom Reid, conservation land management program manager with the Nature Trust of British Columbia (NTBC), who stands atop the 15-foot-high rock embankment he is working to destroy.

The dike, built to fortify farmland stolen from the estuary, is stifling the tidal marsh vital to the survival of a host of endangered salmon and bird species that rely on it for breeding, feeding and migration, he said.

Overhead, a turkey vulture circles, effortlessly riding rising warm air to reach greater and greater heights to scan the mudflats for carrion. But rising seas mean this rich feeding ground will be lost to vultures, and countless other kinds of wildlife, unless action is taken.

But after years of planning and co-ordination, the largest estuary restoration project on Vancouver Island gets underway this summer in a bid to preserve this vital marine and seabird habitat from being eroded by climate change, said Reid.

The scale and significance of the two-year project in Quw’utsun Peoples’ (Cowichan Tribes) traditional territory is tremendously important for the estuary, wildlife and surrounding communities, Reid said.

The estimated $3-million project involves the removal of more than two kilometres of dike and shifting tens of thousands cubic metres of soil to restore and reconnect freshwater channels lost to human development, as well as increasing marshland and riparian habitat along the Koksilah River as it enters Cowichan Bay, he said.

The revitalization of 70 hectares of marsh in the estuary will make it more resilient to sea level rise. Located at the confluence of the Koksilah and Cowichan rivers, which share the same floodplain and tidal marshes, the estuary provides an ecologically rich transition zone between land and ocean.

Estuaries typically rely on the buildup of sediment to maintain themselves, he said, but as global warming drives up sea levels but lowers river levels and the rate of deposits at their mouths, tidal marshes can’t keep pace, he said.

“In the Cowichan, we're seeing a projected sea level rise of about nine and a half millimetres a year, but the accretion is only four millimetres — so we're losing,” Reid said.

At current emissions scenarios, relative sea level rise in the Cowichan Estuary is projected to range from 75 centimetres to a metre by 2100, Reid said.

Swaths of valuable tidal habitat in the estuary could be submerged and lost to the ocean, especially if it can’t expand landward because it’s pinched against increasingly “hardened” shorelines — human development like dikes, seawalls, ports or even new climate construction projects to protect infrastructure and populations from storm surges and rising oceans.

“When it runs up against infrastructure and dikes, you start to drown out those important habitats,” Reid said.

“You get what they call the ‘coastal squeeze’ and you start losing estuaries.”

Nature Trust BC and its partners are removing more than two kilometres of human-created dikes to boost Cowichan Estuary's resilience to climate change.

While estuaries and coastal wetlands comprise less than three per cent of B.C.’s

coastline, they support more than 80 per cent of coastal fish and wildlife, Reid said. Some of the largest and richest estuaries on the south coast have been lost to shoreline development, drained to create agricultural land or dredged to accommodate shipping or the forest industry and log transport, according to B.C. government reports.

Up to 70 per cent of the Fraser River Estuary, at the mouth of the province’s most productive salmon watersheds, has been diked, drained or filled for development. And half of the Cowichan and Nanaimo rivers’ wetlands on Vancouver Island have disappeared.

The Westcan Terminal causeway — a long roadway built in the 1950s to reach a shipping terminal and wood mill — bisects the estuary, altering tidal processes and effectively blocking the southern portion of marsh from needed sediment buildup from the Cowichan River.

Climate change is also expected to stress estuary health, with rising ocean acidification and temperatures and changing salinity, freshwater and sediment levels, Reid said.

On the other hand, the restoration project will boost the region’s resilience to climate impacts — by capturing CO2 and buffering the increasing instances of flood and drought in the Cowichan watershed, he stressed.

Carbon captured and stored in 363 hectares of salt marsh sediment in the Cowichan Estuary is roughly double the amount sequestered by a 20-year-old second-growth forest of the same size, a 2021 University of Victoria study on blue carbon capture in the estuary showed.

Blue carbon — CO2 captured from the atmosphere by marine plants and algae — collects as organic debris in estuary sediments where low-oxygen conditions prevent decomposition and the release of carbon back into the atmosphere, which typically occurs as natural matter breaks down.

Organic carbon buried in the Cowichan Estuary each year, mostly in the top 20 centimetres of sediment, is estimated to be equal to the annual greenhouse gas emissions of 133 vehicles, the study found. Additionally, human activity has reduced the carbon capture and storage capacity of the estuary by about 30 per cent, with 100 hectares lost to farming and 129 hectares of eelgrass habitat disturbed by log-handling and sorting.

Fill removed from the dike removal will be used for natural sloped berms to create a flood fringe forest that will slow storm surges or floodwaters. This will also build up the sediment and add more vegetation and tidal habit, which will serve as a buffer for Cowichan Bay Road, nearby farms and planned trail systems from erosion and water damage, Reid said.

Such natured-based climate solutions also boost biological productivity and lower water temperatures in the estuary, which are particularly important as nurseries for salmon, other fish and Dungeness crab, he said.

Chinook, pink, coho and chum runs, along with steelhead and rainbow trout populations, rely on the health of the estuary, he said.

As many as 230 species, a lot of them migratory seabirds, count on the estuary during the winter or as an important rest stop and fuelling station on those long journeys south or north along the Pacific Flyway, a key migratory route stretching 15,000 kilometres from Alaska in the north to Patagonia in South America.

The estuary supports a sizable colony of great blue herons and other important species such as trumpeter swans and western grebes, at risk of extinction, which overwinter in the tidal zone and nearby surroundings.

The restoration project is a collective effort by the Nature Trust, Cowichan Tribes, Ducks Unlimited Canada, the B.C. and federal governments and the Habitat Conservation Trust, Reid said, noting efforts to protect the area began in the mid-1980s and ensuing land purchases, donations and covenants mean much of the estuary is now protected.

But climate change will completely erase those gains without restoration projects and a large collaborative effort like the one getting underway in August, Reid stressed.

“If we don't do this intervention, we're looking at losing 50 per cent of the marsh habitat in the Cowichan by 2050 and almost 100 per cent by 2100.”

Rochelle Baker / Local Journalism Initiative / Canada’s National Observer