Exploring California’s Marine Protected Areas: Ten Mile Estuary State Marine Conservation Area

Ten Mile River Bridge
Ten Mile River Bridge crossing Ten Mile Estuary State Marine Conservation Area
CDFW photo by L. Lopez

The sand dunes, river, and beach at Ten Mile Beach State Marine Conservation Area (SMCA) are an enticing draw for people staying at MacKerricher State Park. Located off Highway 1 north of Fort Bragg, this marine protected area (MPA) is clearly a special place for beach walks and bird watching.  

What you may not realize is that Ten Mile Beach SMCA has an inland neighbor, Ten Mile Estuary SMCA, that can be found by simply turning away from the ocean to face inland, up the winding Ten Mile River. Ten Mile Estuary SMCA is an entirely different type of MPA, one that is also worth taking the time to explore.

Ten Mile Estuary SMCA is situated east of Ten Mile Beach SMCA, and close to Ten Mile State Marine Reserve. These three MPAs were established in 2012 as part of a network of 124 California MPAs managed by the California Department of Fish and Wildlife (CDFW) and its partners.

This tri-MPA area encompasses beaches, rocky shore, coastal marsh, freshwater marsh, eelgrass, and estuarine habitats. Ten Mile Estuary SMCA regulations do not permit any fishing or other take within its borders, except that waterfowl may be taken in accordance with general waterfowl hunting regulations.

Functions of the Estuary

Ten Mile Estuary SMCA is one of only four estuarine MPAs on California’s north coast. It covers less than a quarter square mile from the mouth of Ten Mile River to the confluence of the Ten Mile and South Fork Ten Mile rivers, but it is an important home and nursery area for ocean fish, crustaceans, and other animals. Several commercially and recreationally important species, including salmon and Dungeness crab, use the estuary during reproduction, and for hunting and feeding grounds. These species, along with certain plants, like rushes, are culturally important to the Northern Pomo and Coast Yuki peoples; for this reason, Ten Mile Estuary SMCA regulations allow for some tribal take.

young Dungeness crab
Young north coast Dungeness crab
photo by A. Young, CC BY-NC 2.0

The estuary also performs an important ecological function by maintaining water quality through natural filtration. Sediments in the estuary bind pollutants, and microbes break down organic matter in the slow-moving wetlands that border the river.

Not every estuary is alike on the Mendocino County coast (where Ten Mile Estuary SMCA is located). For example, recent studies found that Ten Mile River Estuary is more like Mad River Estuary in Humboldt County (about 100 miles north, as the raven flies) than nearby Big River Estuary. Ten Mile and Mad river estuaries are similar because they are both closed off by sandbars for much of the year, and less influenced by salt water. This means the plants, algae, invertebrates, and fish in these estuaries reflect the stronger influence of fresh water. On the other hand, salt water intrudes into nearby Big River Estuary nearly year-round, and the life found there is more oceanic.

Extreme high tides, heavy surf, and stormwater runoff typically breach the sandbar at the mouth of Ten Mile River and keep it open between December and April. When this happens, seawater surges up the estuary and inundates the flood plains on either side of the river. At the same time, winter stormwater runoff from the Ten Mile River watershed can bring a flood of fresh water down river. This increased flow scours sediment from the riverbed and carries it downstream. The exchange of salt and fresh water along with the movement of sediment makes this type of estuary particularly productive, yielding more organic matter per unit area than forest, grassland, or farmland – in essence, capturing carbon dioxide at a higher rate than many other habitats, and storing it in the estuary’s plants, animals, and sediments.

mouth of Ten Mile River
Mouth of the Ten Mile River
photo © 2002-2021 Kenneth & Gabrielle Adelman, California Coastal Records Project, http://www.Californiacoastline.org

The sandbar’s presence or absence produces wildly changing conditions in Ten Mile River Estuary, which fluctuates from a brackish lagoon to a tidal estuary. Plants, fish, and other animals in the estuary have adapted to tolerate wide variations in salinity, temperature, depth, and sedimentation. While some rely on the estuary throughout all stages of growth, development, and reproduction, certain species of fish, birds, and invertebrates only use it for feeding, nesting, and/or breeding during particular parts of their life cycle.

Anadromous Fish at Ten Mile River Estuary
Estuaries are essential to the life cycle of anadromous fish. These fish begin their lives in fresh water, migrate to the ocean to grow and mature, and eventually swim back to spawn in the freshwater stream or river where they were born. Anadromous fish found in Ten Mile River include three species of fish in the salmon family (known as salmonids): Chinook and coho salmon, and steelhead trout. The protection of this essential waterway is important to the health of these salmonids, which are variously listed under both the state and federal Endangered Species Acts.

young Chinook salmon
Young Chinook salmon from the South Fork Ten Mile River
CDFW photo

The Ten Mile River Estuary has been identified as important nursery habitat for coho salmon and steelhead trout. While the life cycles of various salmonids differ in the details, all of these fish spend the early parts of their lives in fresh water, and rely on the estuary environment for food and protection. As older juveniles (known as smolts) migrate to the ocean, they become acclimatized to more saline water due to the mixing of ocean and fresh water in the estuary.

After spending one to four years in the ocean, adult salmonids return to spawn in freshwater streams and rivers. Coho and Chinook salmon die after spawning, while steelhead trout can spawn up to three times over multiple years. Because of their complex life cycles, salmonids pass through Ten Mile Estuary SMCA at least twice during their lifetimes.

CDFW has partnered with The Nature Conservancy to restore essential habitat in the South Fork Ten Mile River, just upstream from Ten Mile Estuary SMCA. This restoration work is necessary after decades of logging, gravel mining, and cattle grazing. “The ultimate goal, with time and more projects, would be to work towards improving the natural function of the river, including appropriate sediment movement and deposition that should improve estuary habitat, creating an ecosystem that is more balanced and supports a diversity of species,” said Sarah Gallagher, a CDFW senior environmental scientist working on the salmonid habitat restoration project.

Enjoying the Trails near Ten Mile Estuary SMCA
There are several ways to enjoy Ten Mile Estuary SMCA. Small parking lots on both the west and east side of Highway 1 (as you approach Ten Mile River Bridge from the south) provide places to park and enjoy views of the SMCA.

path across Ten Mile River Bridge
Accessible path across the Ten Mile River Bridge
CDFW photo by L. Kashiwada

The parking lot on the west side is located next to Inglenook Fen-Ten Mile Dunes Natural Preserve, at the northern portion of MacKerricher State Park. Interpretive panels here describe the three MPAs in the local area. There are good views of Ten Mile River Estuary both east and west from the Ten Mile River Bridge, which has a wheelchair-accessible path across it. However, there is no access to the river itself from the west side of Highway 1.

The parking lot on the east side of Highway 1 provides access to the Old Smith Ranch Trail, part of the Mendocino Land Trust trail system, which opened to the public in July 2020 for hiking and bird watching. The trail winds west down a hill towards Ten Mile River Bridge, then takes a hairpin turn to the east and follows Ten Mile River as it meanders between the banks of low-lying flood plains. Inundated with seawater in the winter months, the plains were used for cattle grazing in the not-so-distant past. The terrain steepens quickly on the wooded south side of the river, while the rolling hills on the north side are still used for ranching.

There are several interpretive panels along the trail, including one that relates the history of logging in the Ten Mile River watershed. The trail runs along the roadbed of the old railroad that used to haul logs to the mill. This trail is part of the larger Smith-Perry Ranch and is preserved in perpetuity for public enjoyment. Dogs are not allowed on this trail and there is no direct access to the river. Also note that the land on the north side of Ten Mile River is privately owned and there is no public access to the river there.

Although direct access to the water is very limited, Ten Mile Estuary SMCA is a wonderful place to hike, birdwatch, and take in the scenery using one of the points of access described above.

Remember to check for current local COVID-19 restrictions before you visit Ten Mile Estuary SMCA.

Ten Mile Estuary State Marine Conservation Area is one of the 124 MPAs in California’s statewide MPA Network. Please visit CDFW’s MPA website for more information, and sign up to receive updates about the MPA Management Program.

MPA logopost by Leslie Kashiwada, CDFW Scientific Aid


Learn more about MPAs by diving into the
Exploring California’s Marine Protected Areas series!