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N Spit Coquille - mild saltpan - 2017-08-15 MGiles

The pattern of plants in this bit of estuarine marsh community on the north spit of the Coquille River near Bandon indicates a variety of micro-habitats. The low area in the center is a salt pan, where a pool of salt water was trapped: as the water evaporated, the pool got saltier.  

Not raining, raining. Raining, not raining.

We pay attention to rain so we can select the proper clothing, decide when to do certain outdoor activities, such as going for a hike or mowing the lawn, or remember to check the patch on the roof.

A lot of rain saturates the ground and can overwhelm watercourses to flood the landscape. But even “normal” amount of rain we get in winter has a significant effect on the estuaries.

Estuaries are places where fresh water and salt water meet and mix. Coos Bay and other Oregon river mouths, such as the Siuslaw, Umpqua, and Coquille, are estuaries. Coos Bay is the largest estuary entirely in Oregon since we share the region’s largest estuary, the lower Columbia River, with Washington.

The fresh water of estuaries is precipitation that collects in the watershed and eventually arrives as the rivers’ and streams’ current. The salt water is drawn into the estuary by the incoming tide and subsides with the ebbing tide. On the whole, the upper end of the estuary is fresher than the lower end; the lower end of the estuary is saltier than the upper end.

Because our climate features wet winters and dry summers, the mix of fresh and salt in any particular place in an estuary varies with the season, as well as with the tide. Some places in our estuaries that can be as fresh as a stream at low tide during heavy winter flows can also be nearly as salty as the ocean during high tides at the end of a dry summer.

Of course, the tide raises and drops the level of water in the estuaries. How often a particular place is exposed/covered plays a key role in what goes on there.

The channel is the deepest part of the estuary, where the bottom’s always covered by water. The open waters of the channel harbor fish, seals, and other large mobile animals, as well as the microscopic plants and animals that make the water murky with life.

Just above the channel is the tideflat. How much tideflat is exposed varies with the tide, but all the tideflats are underwater too much to support rooted plants. (“Tideflat” is a better choice overall than “mudflat” to describe this habitat since such areas can be floored with sand, gravel, or rock, as well as mud.) Many of the denizens of tideflats are burrowing animals, such as worms and clams. Clammers know that different kinds of clams are more or less likely in different places -- places that are fresher/saltier, sandier/muddier, more/less often uncovered by the tide.

Higher than the tideflats are the marshes that are uncovered by water more often than covered; it takes a very high tide for the fresh/salt water to be driven high into the marshes.

Each habitat -- delineated by exposure by the tide (channel, tideflat, marsh) and by salinity of the water -- supports its own community of organisms. The currents and tides mix the waters and transport floating plants and animals around the different habitats of the estuary.

It is the marshes that most readily show the variety of salinities and inundation. At one extreme of salinity, lanky bulrushes thrive at the upper limit of the saltwater influence and cattails grow just above, in all fresh water. At the other extreme, the saltiest marshes near the estuary’s mouth appear close-cropped, dominated by the short pickleweed and saltgrass.

Estuarine marsh communities also vary by differences in elevation, with the marsh plants thinning out as the lower edge of the marsh gives way to the tideflat. Tolerant upland shrubs edge into the upper margin of the high marsh that’s rarely flooded by salt water. Still finer patterns form in the marsh based on salinity and tide-driven inundation. Sometimes a huge variation of communities can be seen in a small area, even with only an inch or two difference in elevation.

The high tide that brings the salt water, also brings salt-loving animals, such as crabs, into the estuary, and delivers fresh organic material that fertilizes the marsh. In turn, the marsh thrives, leveraging that fertilizer for growth that eventually enriches the whole estuary. Like other wetlands, estuarine marshes buffer shorelines, collect and store water, clean water, and are critical habitat for wildlife.

Judging by the sheer mass of life, estuaries are among the most productive places on the planet.

So estuaries are all about in-between. They’re in-between fresh and salt, and in-between deep and shallow. The default shape for Pacific Northwest estuaries is a drowned river mouth, with a meandering channel flanked by tideflats, edged with fringing marshes.

We don’t like the in-between stuff, however, and would rather have the water very deep for our vessels and limited standing water on the land for our farms and buildings and roads. Of course, that’s what dredging is all about: make the channel deeper for boats and, historically, make the land higher/dryer for land uses. The easy way to meet both those ends is to use the dredge spoils to fill the marshes -- and until rather recent conservation laws, that’s what we did. Pretty much every place that’s flat and near the water where there’s a tide, either is -- or used to be -- a tideflat or marsh.

Wet winter, dry summer; high tide, low tide; channel, tideflat, marsh. The myriad environments of the estuary are complex, dynamic, and interactive, fluctuating with the tides and seasons, and sometimes changing over time.

For information on how you can arrange an exploration of our fascinating natural history, contact Marty at 541-267-4027,, or Questions and comments about local natural history are welcome.

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