Gradually, 33 tons of hand-carried rocks coalesce into a wall about knee high and nearly 200 feet long. It arcs along a contour line 2 feet below the typical low tide. Most of the time, the wall will be submerged, invisible to visitors to the Kukutali Preserve, co-managed by the Swinomish Tribe and Washington State Parks.
Over time, the sturdy but porous structure should capture sediment on its upland side and expand the shallow, gently sloping habitat for things like butter clams and littleneck clams.
As with any backyard garden, continual tending—in this case, by clearing rocks and algae from the clam-growing areas and digging into the sediment with sticks to aerate it—will be part of ensuring a productive harvest.
Clam gardens grow four times more butter clams and twice as many littleneck clams as unterraced beaches do, according to a study of dozens of ancient clam gardens around Quadra Island, British Columbia. Young littleneck clams planted in the centuries-old terraces grew nearly twice as fast, making more local protein available to shellfish harvesters.
[...] Crushed shell bits are expected to wash in and pile up behind the wall.
They can locally neutralize some of carbon dioxide that’s making sea water more acidic and less hospitable to shellfish as well as overheating the planet.
Some more information on clam garden designs:
Sequence of construction of three different clam garden forms, based on northern Quadra Island sites. Falling sea level during the Holocene means that the base of many clam garden walls, particularly those built on soft sediment, are often exposed during the lowest tidal windows today. Form 1. Clam gardens built on soft sediment beaches; (1a) Original unmodified beach surface with existing clam habitat prior to wall construction; (1b) Clam garden wall and terrace on same beach. Note clams and barnacle scars from original beach covered by clam garden wall and terrace sediment. Note also some boulders with barnacle scars from original beach moved into clam garden wall during construction. Stippled light coloured sediment behind wall indicates shell hash and coarse sediment that accumulated after wall was built, thereby expanding clam habitat; Form 2. Clam gardens built on bedrock shelf; (2a) Original bedrock shoreline; (2b) Wall built on same bedrock to create clam garden terrace. Note base of wall is not always accessible at low tide; Form 3. Clam gardens built on steep eroding bedrock shoreline; (3a) Original steep, eroding bedrock shoreline. (3b) Clam garden wall and terrace in same location. Rocks from mid to upper intertidal and supratidal are moved down slope to create levelled terrace at the elevation for optimal clam habitat. Note base of wall is usually not accessible at low tide.
Forgot to add this bit of info, from an article before they were legally given the right to build the clam wall:
Just as with a vegetable garden, Hatch says, clam gardens require regular care and demonstrate that people can interact with and harvest from nature in a mutually beneficial way. Tribes maintain the rock terrace as it weathers storms and waves. They remove seaweed, and dig clams strategically, selecting larger clams and making space for smaller ones. If they don’t till the beach, it can get too compact and low in oxygen for baby clams to take root. “People often refer to that as fluffing the beach,” Hatch says.
[...] Clams thrive in areas with a high stream flow, a mix of fine sediment, pebbles and shells, gentle wave energy and enough food.