Types of estuaries in Puget Sound

An estuary is a place where saltwater from the ocean mixes with freshwater from rivers and streams. Technically, this defines all of Puget Sound, but scientists have identified several types of "sub-estuaries" within the water body. These include pocket estuaries (or embayments), tidally-influenced rivers and wetlands and other areas near the shoreline connected with freshwater sources. This summary provides descriptions of these estuaries from the Washington State Department of Natural Resources, the Puget Sound Nearshore Partnership and others.

Tidal marsh at the Nisqually National Wildlife Refuge in Puget Sound. Photo courtesy of USFWS.

Editor's note: To understand the types of estuaries within Puget Sound, we must first come to a common understanding of Puget Sound's boundaries. For the purposes of this article, Puget Sound includes the Puget Sound Basin that stretches from the waters from Admiralty Inlet and Deception Pass to the southern tip of Olympia (see: Geographic boundaries of Puget Sound and the Salish Sea). Additional estuarine waters include the Strait of Juan de Fuca and areas northward into the Salish Sea including the Strait of Georgia. These waters make up a broader estuarine complex that is connected to and includes Puget Sound.

The following text is taken from “A Marine and Estuarine Habitat Classification System for Washington State” by Megan Dethier, prepared for the Washington State Department of Natural Resources.

Overview: The estuarine system within Puget Sound

The Estuarine System generally consists of waters that are semi-enclosed by land but have open, partly obstructed, or sporadic access to the ocean, and in which seawater is at least occasionally diluted by freshwater runoff from land. It extends upstream and landward to where ocean-derived salts near the water surface measure ~0.5 parts per thousand (ppt) during the period of average annual low flow, and downstream or out to sea to where freshwater dilution is minimal (salinities seldom falling below 30 ppt). The Estuarine System thus includes classic river-mouth estuaries, lagoons, and large bodies of water such as Puget Sound and the Strait of Georgia which are signficantly diluted by freshwater input from numerous sources.

Many areas in the Puget Trough are difficult to categorize as either estuarine or marine. In these transition areas, salinities are generally high (>25 ppt) and the assemblages resemble those in truly marine waters, but surface salinities occasionally drop lower than this, and circulation and nutrient levels are influenced by estuarine processes. The San Juan Islands fall into this category. Extensive turbulent mixing, combined with strong tidal flow from Juan de Fuca Strait into deep, narrow channels (Thompson and Robinson, 1934; Thomson, 1981) generally keep salinities and nutrient levels high. Most regional scientists consider the flora and fauna to be marine. However, the strong influence of the Fraser River from the north occasionally causes large drops in surface salinities, and areas to the east are influenced by freshwater runoff into Bellingham, Padilla, and Skagit Bays.

Rather than rely on a purely salinity-based marine/estuarine cutoff for this area (which would require arbitrary definition not only of a salinity level but also a frequency for this level of intrusion), we have drawn a geographic boundary for this transition through Rosario Strait. Areas to the east of a line from Green Point (Fidalgo Island) to Lawrence Point (Orcas Island) are considered estuarine, as are all of the Strait of Georgia and the San Juans north of Orcas. Areas to the west are marine (including islands on both sides of Haro Strait), as is the west side of Whidbey Island down to Admiralty Head. To the east of Deception Pass, and to the south and east of Admiralty Head (and south of Point Wilson on the Quimper Peninsula) is Puget Sound proper, which is considered to be entirely estuarine. Deeper waters (> approx. 50 m) in all these areas are quite "marine" in terms of both salinities and assemblages, but are defined as estuarine along with their accompanying shallower zones.

In the "marine" areas outside this invisible line there clearly exist some estuarine pockets: Dungeness Bay (Dungeness River), Sequim Bay (enclosed, with stream input), the outer coast estuaries of Grays Harbor and Willapa Bay, and perhaps others. Discovery Bay, which is relatively open and has little freshwater input, is considered marine.

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Estuarine classifications

Portions of the following text are taken from “A Geomorphic Classification of Puget Sound Nearshore Landforms” by Hugh Shipman, prepared in support of the Puget Sound Nearshore Partnership.

Pocket estuaries, or embayments

Embayments: This term describes protected estuaries and lagoons within which there is too little wave action to form beaches. The term pocket estuary has been widely used in Puget Sound to describe these features. Most of these small embayments are tidally influenced, but they also include isolated lagoons and wetlands. Estuaries are those with a significant input of freshwater – for example, from a surface stream, whereas lagoons have limited freshwater input. A large number of the estuaries and lagoons on Puget Sound are formed and enclosed by barrier beaches, emphasizing an important geomorphological relationship between the wave-dominated beach environments and these small protected estuarine environments.

Large river deltas

Large river deltas: This category is reserved for the deltas of the large rivers that drain the Cascade and Olympic mountains. These deltas, built of fluvial sediment deposited at the coastline, are often broad and low-lying, and represent the marine extension of alluvial floodplains. Many have been heavily modified for agricultural and urban uses. Deltas can be distinguished based on the rela-tive influence of waves, tides and river processes in their formation. Deltas are associated with streams of all sizes, but smaller ones are generally subsumed within the other systems – for example, stream deltas can occur within the upper reach of a small estuary or directly on an exposed beach.

The broader estuarine complex

General mixing throughout the system: The Puget Sound basin is connected to the Strait of Juan de Fuca and the Strait of Georgia as part of the broader Salish Sea. The entirety of this region can be described as an estuarine complex with variations in freshwater mixing throughout. In some cases this mixing varies according to nearshore features such as beaches or rocky shorelines. Because estuarine waters are often connected and boundaries may be fluid, additional categorizations of some estuaries may occur in more detail than we have described here. Estuarine waters can also vary by season. Surface waters through the Strait of Juan de Fuca technically vary seasonally between estuarine and marine, relative to "polyhaline" salinities 18-30 ppt/psu (Ebbesmeyer et al. 1988).

Citations:

Ebbesmeyer, C. C., J. Q. Word, and C. A. Barnes (1988): Puget Sound: a fjord system homogenized with water recycled over sills by tidal mixing. Hydrodynamics of Estuaries: II Estuarine Case Studies, B. Kjerfve, Ed., CRC Press, 17-30.

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