How Long Will It Take For Tidal Wetlands To Develop?

Mist rising from Lagunitas Creek © Galen Leeds Photography
Mist rising from Lagunitas Creek near White House Pool.

Because pasture elevations are relatively similar to undiked areas, the transition to tidal marsh was predicted to be relatively quick, particularly compared to many wetland restoration projects in San Francisco Bay.

The speed of this transition was expected to be dictated to some degree by how quickly the pasture grasses in Giacomini Ranch died off. The pasture grasses that had established in the ranch consisted of species with dense roots, rhizomes (spreading, underground stems), and stolons (spreading, aboveground stems). These grasses, which are somewhat tolerant of saline conditions, would be expected to resist encroachment by new species.

Based on these and other factors, it was originally estimated that conversion to natural tidal marsh would take a minimum of 10 to 20 years-possibly longer-and would involve establishment of some transitional habitats (see poster for more information on transitional habitats). However, these expectations were shifted somewhat recently, because, even prior to removal of the levees, rapid conversion of habitats was already occurring. This conversion seemingly resulted from discontinuation of agricultural management-including irrigation of the root zone with freshwater in the summer-and several incidental levee breaches prior to construction. There was a strong resurgence in native grasses such as wildrye and meadow barley and the extent of transitional habitat and even salt marsh increased greatly. Because of this, last fall, we readjusted our estimates on how quickly tidal marsh would develop to as little as 5 to 10 years.

These estimates on the "successional trajectory" and timeline of habitat evolution are predicated on a Gradual Continuum Model of ecosystem evolution. This model assumes a linear model of change from the disturbed condition (diked pasture) to a more natural state (tidal marsh) with removal of the levees, tidegates, culverts, ditches, and other hydrologic and topographic constraints. While basic and restoration science often operate in seemingly different spheres, predictions on future changes in habitat made by practitioners of restoration science and management often draw from basic ecological understanding of how physical and biological communities change. One of the most famous theoretical constructs of ecological change is the so-called Clementsian succession model, in which forests convert to meadow during fires and then evolve over time back to forest, the "climax" community, through successional processes.

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While not necessarily envisioned as a classic successional process, the evolution of the Giacomini Wetlands was anticipated to occur more or less in a linear fashion or along some type of continuum of change (i.e., A→AB→B). During planning, the trajectory or endpoint for habitat evolution at the Giacomini Wetlands always remained the same. Disturbed conditions or pasture would convert to more natural conditions or marsh. The timeline was simply changed to reflect a shorter timeframe between breaching and expected endpoint conditions.

Since breaching of the levees, however, it is apparent that the transition to tidal marsh will not necessarily follow this Gradual Continuum Model of habitat evolution, but more of what is being called a Threshold or Dynamic Regime Model (Hobbs and Suding 2008). While certain threshold-type models also assume progression from a disturbed condition to a more natural state with removal of disturbance factors, the progression is not necessarily assumed to be linear or to occur along a continuum of conditions. Rather, under these models, change occurs in a more discrete, step-like fashion, often with abrupt transitions between different states or conditions that require certain "thresholds" to be passed for movement from one state or condition to the next.

In terms of the Giacomini Wetlands, the factors discussed under "Water, Water, and More Water" that are causing more extensive subtidal conditions in the East Pasture and, to a lesser extent, in the West Pasture than predicted originally under levee removal computer modeling scenarios are also affecting successional trajectories. As was discussed earlier, permanent ponding of waters during even very low tide conditions is occurring currently in a greater percentage of the wetlands than expected due to the fact that inflow substantially exceeds outflow. This differential in inflow and outflow volume results from several factors, including: 1) created tidal inlets are not wide enough currently to accommodate inflows/outflows; 2) additional tidal channels have not evolved yet in the new marshplains to more expeditiously convey flow to larger tidal sloughs and channels; and 3) high elevation marsh shelves outboard of the former levees are acting as "mini" levees and precluding waters from draining back into Lagunitas Creek. Many of these factors were anticipated: the created tidal channels were intended more as templates or "starter" channels, with final evolution of smaller channels and morphology or dimensions of larger channels expected to develop naturally over time.

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Figure 01 - Trajectories of Habitat Evolution: Giacomini Ranch - East & West Pastures. Click on this image to download full size version of this chart. (20 KB PDF)
Figure 1: Trajectories of Habitat Evolution: Giacomini Ranch - East & West Pastures.

Because of these factors, the wetlands, rather than progressing linearly on a continuum from diked pasture to tidal marsh, are evolving first into a slightly different state or condition characterized by more extensive ponding under subtidal conditions. This different state or condition is depicted in the graphics in Figure 1: Trajectories of Habitat Evolution: Giacomini Ranch - East & West Pastures (20 KB PDF), which compares the two different models of habitat or marsh evolution. As is shown in Figure 1, the change in successional trajectory has distinct implications for evolution of the system. The extent of intertidal marsh vegetation will be reduced under the Phase I Trajectory, and, so, to some extent, will be development of intertidal mudflats. The larger expanse of open water areas will benefit waterfowl, particularly certain types or groups of waterfowl species such as dabbling ducks, and perhaps encourage more use by non-resident or transient fish species, including some of the larger, open water species such as topsmelt.

Over time, however, this state or condition will continue to change or evolve, as created tidal inlets widen naturally to better accommodate inflows/outflows; additional tidal channels evolve naturally in the marshplain to better convey flows to sloughs; and outboard marsh shelves naturally erode. These changes will start to move this system closer to the original successional or evolution trajectory of tidal marsh (Phase II Trajectory). The transition between the Open Water/Tidal Marsh and Tidal Marsh systems or conditions is expected to change more slowly in the East Pasture than in the West Pasture, because the dense, clay substrate in the East Pasture will slow down the process of inlet widening and natural channel evolution. The West Pasture is underlain by coarser soils that are expected to allow for faster response in inlet widening and channel development to the new hydrologic regimes.

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At a certain point, a "threshold" condition will be crossed, probably during a season with high stormflow that encourages loss of the precarious high-elevation outboard marsh shelves. Crossing of this threshold will result in increased drainage of the new wetlands during extreme low tide conditions to a degree more in keeping with that originally predicted by computer modeling and will begin to move the system towards a new "state" or "condition"-tidal marsh system characterized by a lower extent of open water, but increased intertidal marsh and mudflat. This system will have potentially greater benefits for shorebirds and resident marsh bird species, but may have fewer benefits for waterfowl and non-resident and larger estuarine fish species.

How long for this evolution to take place?

It is difficult to predict, because some of the factors such as loss of the "mini" levees need to trigger transition between the two states rely on climatic or weather conditions such as high levels of rainfall accompanied by surges in stormwater flow. So far, the winter of 2008-2009 has been relatively dry, with little increase in water levels during the few storms that have occurred. Other factors such as widening of the channel inlets and channel evolution are driven more by the energy of tidal flows, although they can also be shaped to some degree by stormflows, as well. Based on these factors, the transition could occur in as little as two to three years or as many as five.

Ultimately, this project will provide an invaluable insight into the issues that affect the path or trajectory of restoration of degraded wetlands back to non-degraded conditions-insights that will prove valuable for planning of future restoration projects. In addition, it will help ecologists to better understand the processes of change for all communities, whether they be part of a restoration project or not.

-- Content for this page was composed by Lorraine Parsons, Project Manager, Giacomini Wetland Restoration Project, Point Reyes National Seashore

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Last updated: February 5, 2024

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