Giacomini Wetland Restoration Project: Restoration: Water, Water, and More Water: The Newly Restored System Continues to Evolve Hydrologically
Updated October 2012
This change was predicted. However, what was anticipated less was the evolutionary process by which hydrology within the restored Ranch would evolve, similar to that of vegetation. During planning for the restoration project, computer hydraulic modeling conducted as part of planning for the restoration project estimated that, based on existing and proposed elevations, 256 of the 550 acres in the East (area adjacent to Point Reyes Station) and West (area adjacent to Inverness Park) Pastures of the Giacomini Ranch would be inundated by tides daily or close to daily (KHE 2006). This modeling assumed no levees remained and that there was some tidal channel re-creation. The estimated high water extent, which represents slightly less than half of the total Project Area, falls at or below Mean High Water elevations ranging from 5.0 to 5.1 feet NAVD88 (KHE 2006). During higher high tides, which occur roughly 6.5% of the time, approximately 310 acres below elevations ranging from 5.7 to 5.8 feet NAVD88 were predicted to be inundated by Tomales Bay waters (KHE 2006). Very infrequently, during extreme high tides, as has been observed in mid-December and early January, approximately 382 acres might be expected to be underwater where elevations fall below 7.0 to 7.1 feet NAVD88 (KHE 2006; Figure 1 - 940 KB PDF).
In terms of average and low tide conditions, approximately 63 acres below the Mean Sea Level elevation of 3.5 feet NAVD88 were estimated to be inundated 65% of the time, only being exposed during lower low tide conditions (KHE 2006). This represented approximately 11.5% of the Giacomini Wetlands area.
The model originally predicted that approximately 10.0 acres or less than 1.8% of the Project Area-representing primarily the bottoms of tidal and stream channels-would be permanently inundated or subtidal (KHE unpub. data; Figure 1 - 940 KB PDF). However, later modeling efforts recognized that low water levels would be driven not only by topography, but by height of gravel bars and shoals in the main creek channel that Giacomini Ranch waters would drain to, Lagunitas Creek. While subtidal areas outside of the Giacomini Ranch fall below 0.49 feet NAVD88, computer modeling predicted that subtidal elevations would range from as high as 3.4 feet NAVD88 in the West Pasture to 2.0 ft. NAVD88 in the East Pasture (KHE 2006). These differences result from the fact that two separate gravel bars control minimum water surface level elevations or the extent of subtidal areas for the East and West Pastures by acting as natural "weirs" or dams (KHE 2006). One gravel bar system is north of the Giacomini Ranch in Lagunitas Creek just at and slightly north of the former North Levee in the West Pasture. This bar, which tops out at around 2 feet NAVD88, controls minimum water surface elevations and, therefore, subtidal conditions in Lagunitas Creek and the East Pasture. A separate gravel bar just north of where the culvert for Fish Hatchery Creek used to be controls minimum water levels or subtidal conditions in the West Pasture: the top of this bar is approximately 3.4 feet NAVD88. These bars increase the volume of water retained permanently under subtidal conditions, with subtidal areas predicted at 28.7 acres, almost three times the area predicted under earlier modeling efforts (KHE 2006).
During the first few months after breaching, Park Service staff monitored and mapped tide levels during higher high tide and low tide events to determine how well water levels in the Project Area corresponded with those predicted by hydraulic modeling. Additional monitoring was conducted in December 2009 of extreme high tides and low tides during neap tide conditions, with follow-up monitoring of low tides during spring tide conditions conducted in 2010, 2011, and 2012, and extreme high tides in 2011.
During 2008, 2009, and 2011, the upper extent of higher high tides appearedto match very well with that predicted by computer modeling, often mimicking the contour patterns exactly (Figure 2 - 220 KB PDF). In winter 2008, during tides ranging from 6.3 feet to 6.6 feet MLLW (5.8 feet to 6.1 feet NAVD88), tidewaters extended up to the 7.0 to 7.5 feet NAVD88 elevations in the East Pasture (NPS, unpub. data). Computer modeling predictions estimated maximum tidal elevations in the East Pasture of 7.15 feet NAVD88, with the area exposed to tidal influence at extreme tides being close to 282 acres (KHE 2006; Figure 1 - 940 KB PDF). However, while the maximum tidal elevations were sometimes greater than predicted, the amount of area in the East Pasture subject to extreme tidal influence turned out to be slightly lower than that predicted (251.7 acres), a difference of 11 percent (NPS, unpub. data; (Figure 2 - 220 KB PDF). In 2009, the acreage of areas subject to high tides was only slightly less (242.5 acres), which probably resulted largely from the fact that mapping was conducted during an approximately 1 foot lower tide level (5.8 feet to 6.3 feet MLLW or 5.3 feet to 5.8 feet NAVD88; (Figure 2 - 220 KB PDF). Otherwise, there appeared to be good agreement between 2008 and 2009 high tide extent.
In the East Pasture, tidewaters often follow the approximate contours of grading of the Marshplain Enhancement area, a 16-acre area in the southwestern portion of the East Pasture that was lowered 1 to 2 feet in elevation. In addition, they extend just slightly upstream of the area where Tomasini Creek was rerouted out of its formerly leveed alignment to create Tomasini Slough. Figure 3 (52 KB PDF) shows minimal water fluctuations in upper Tomasini Slough prior to breaching of the levee and the corresponding change in water levels and water level fluctuations with the accidental breaching of the levee in early July 2008 and early November 2008, after the levees were purposefully breached. As designed, during the extreme high tides, tidewaters extended up to the very base of the berm for the Tomasini Triangle Marsh.
The maximum elevations reached by extreme high tides were a little more variable in the West Pasture. During tides ranging from 6.3 feet to 6.6 feet MLLW (5.8 feet to 6.1 feet NAVD88), tidewaters extended up to the 5.75 feet to 7.0 feet NAVD88 elevations in the West Pasture (NPS, unpub. data; (Figure 2 - 220 KB PDF). During extreme tides, tidal waters were predicted to reach a maximum elevation of 7.13 feet NAVD88 (KHE 2006; Figure 1 - 940 KB PDF). Some attenuation or drop in tidal amplitude would typically be expected as tidewaters crest the banks of tidal creeks and spread across the marshplains or former pastures, although there actually appeared to be amplification relative to the predicted tide level in both the West and East Pastures. The variability in elevation extent in the West Pasture may result from the influences of freshwater, which may "push back" tidal influence where freshwater surface or groundwater flow is strongest. While the southern extent of the West Pasture was surveyed prior to any major rains in 2008, some of the surveying in the central and northern portions of the West Pasture near Fish Hatchery Creek did take place after some rain, and it was near Fish Hatchery Creek that maximum tidal elevations were lowest (5.75 feet NAVD88; 2008 Line in (Figure 2 - 220 KB PDF). Extreme high and low tide lines were mapped using salinity of waters as a guide, particularly after rains and groundwater flow increased freshwater influence. Despite the variable and generally lower maximum tidal elevations in the West Pasture, there was only an 8 percent difference between predicted extent of area inundated at maximum tides (98 acres; KHE 2006) and actual area in 2008 (90.7 acres; NPS, unpub. data). Interestingly, in 2009, the extent of area subject to high tides actually increased very slightly-91.9 acres-even though the restored wetland was surveyed under lower high tide conditions than in 2008 (NPS, unpub. data).
Where computer modeling predictions and actual water levels diverge substantially was with low tide conditions. While mapped high tide water levels correspond pretty closely with predicted levels, water levels during lower low tides are much higher than predicted by computer modeling. In other words, more area is remaining subtidal than predicted. Mapping Mapping of the water levels immediately after restoration during predicted lower low water conditions that ranged from -1.7 feet to -0.4 feet MLLW (-1.2 feet to +0.1 feet NAVD88) showed that minimum water level doubled in the East Pasture (4 feet NAVD88) and increased slightly in the West Pasture (3.75 feet to slightly below 4 feet NAVD88; Figure 4 (347 PDF)). As discussed earlier, pre-restoration subtidal water levels in these areas again were 2.0 feet NAVD88 in the East Pasture and 3.4 feet NAVD88 in the West Pasture (KHE 2006).
In the East Pasture, this increase in minimum water level immediately after restoration represented a substantial increase in the areal extent of permanent inundation from 26.5 acres using predicted subtidal elevations (2.0 feet NAVD88) to 109.4 acres under actual low tide conditions (NPS, unpub. data; Figure 1 (940 KB PDF) and Figure 4 (347 PDF)). This difference constituted an increase in subtidal areas from approximately 7.6% to 31% of the 350-acre East Pasture. The discrepancy was not quite so great in the West Pasture, where predicted extent of permanent inundation (2.2 acres; KHE 2006) or 1% of the West Pasture was only slightly lower than the actual extent of subtidal area immediately after the levees were breached (7.4 acres; NPS, unpub. data) or 3.7% of the West Pasture (Figure 1 (940 KB PDF) and Figure 4 (347 PDF)). Based on mapping conducted in December 2009, the extent of subtidal areas was actually lower under neap tide conditions-when tidal range between low and high tides is substantially reduced-than under spring tide conditions, when low tides reach some of their lowest levels. In December 2009, subtidal areas totaled 52.9 acres in the East Pasture and 4.5 acres in the West Pasture when tides ranged between 1.9 and 2.7 feet MLLW (1.4 and 2.2 feet NAVD88). This represented almost a 51% reduction in subtidal area with an approximately a 1- to at most 3-foot difference in tidal water elevation. These results suggested that drainage was being constrained by the larger volume of water that flows into the newly restored wetland on a spring tide, when high tides are very high, than on a neap tide, when high tides are lower.
Most of the subtidal ponding occurs in the northern portion of the Project Area, where elevations are the lowest, typically because of subsidence or a drop in elevations due to construction of levees and drying out and subsequent compaction of soils. In the West Pasture, persistent ponding occurs in Fish Hatchery Creek and West Pasture Old Slough, as well as in the northeastern portion of the Project Area where the borrow ditch was removed despite efforts to deconstruct in such a way as to not create a depressional basin. Some areas in the southern portion of the West Pasture are functioning more as residual basins of higher salinity waters that flood during high tides and subsequently pond and are, therefore, not necessarily representative of "subtidal" conditions.
In the East Pasture, flooding is particularly persistent in the Shallow Shorebird area adjacent to the Point Reyes Mesa, which is the lowest pasture or marshplain area in the Project Area. The elevation gradient in this area is such that tidal overflow from Tomasini Slough drains eastward to the lowest elevations in the East Pasture adjacent to the Point Reyes Mesa and the levee of the former Tomasini Creek channel. Here, waters pond, because there are no channels or outlets that would allow drainage on low tides.
So, what was causing these higher than predicted subtidal water levels, particularly in the East Pasture? One factor is that, while levees have been removed, the undiked marsh that had developed on the outboard of the levees is, in many cases, higher in elevation than the marshplains or former pastures. These marsh shelves, then, are acting as mini "levees" and not allowing most of the tidal waters to drain westward into Lagunitas Creek, particularly in the northern portion of the East Pasture. The model assumed that there would be no levees present. Because of these mini-levees, waters are being funneled exclusively through the two primary tidal channel outlets that were created-the Tomasini Slough, which flows into Lagunitas Creek near Railroad Point in the northern portion of the East Pasture, and the new side channel for Lagunitas Creek, which drains the new Marshplain Enhancement area in the southwestern portion of the East Pasture.
Extremely strong tidal velocities at the mouth of Tomasini Slough indicated that tidal channels were still adjusting to the volume of tidal waters that enter during high tides, with velocities expected to drop somewhat when the width of the channel becomes large enough to fully accommodate flows. Larger tidal channels were deliberately constructed as templates to allow for more natural channel evolution. In addition, only a few smaller tidal creek channels were constructed, with the assumption that other smaller channels would develop naturally over time. Because channel width and density has not been large enough to fully accommodate flows, waters have not been fully draining on the low tide, leaving the amount of open water area in the East Pasture larger than would be expected based on elevation alone. Also, the lack of vegetation that occurred when pasture vegetation rapidly died after levee breaching encouraged overflow of tidal waters and floodwaters onto marshplain rather than keeping them in channels, which slows down the morphological evolution of larger channels (KHE 2010). This also accounts for the reduced extent of subtidal areas during neap relative to spring tides and the large lag observed in the peak low tide conditions, which were often delayed beyond predicted low tide times at Inverness by as much as 3 hours or more. Interestingly, at tide levels below 3.7 feet NAVD88, the existing tidal channel network appeared to keep pace with inflow of waters from Lagunitas Creek: it was at tides larger than 3.7 feet NAVD88 that inflow and outflow appeared to become out of synch with Lagunitas Creek and Tomales Bay (KHE 2010).
The effect of marsh drainage patterns was also evident in Lagunitas Creek, where continuous water quality monitoring conducted by Kamman Hydrology & Engineering both prior to and immediately after breaching of the East Pasture levees documented that the salinity range in Lagunitas Creek increased from between 10 and 32 psu immediately pre-restoration to between 18 and 34 psu immediately after restoration (KHE 2009a; Figure 5 - 407 KB PDF). One year later, the range of salinities during a similar fall monitoring period in 2009 to pre-restoration monitoring conducted in September 2008 showed a compressed range of salinities, with higher lows and lower highs (KHE 2010). In addition to changes in salinity, water level patterns at the former North Levee in Lagunitas Creek also showed some flattening of the water level curve as water levels drained below 3.5 feet, suggesting that water levels were dropping more slowly because of the added volume it had to convey from the marshplain (KHE 2009a).
Circulation and drainage patterns are being altered after restoration by changes in Lagunitas Creek and interior tidal channel geometry. Immediate post-project surveys had indicated a uniform increase (1.0 ft) in bed elevation of the mainstem Lagunitas Creek channel immediately upstream of the former cattle crossing near White House Pool in 2009 relative to elevations in 2003 (KHE 2009b). In contrast, channel elevations immediately upstream of the former North Levee area remained fairly comparable in 2009 to those measured in 2003 by the USGS, (KHE 2009b). Since restoration, elevations within the Lagunitas Creek cross-sections have not changed appreciably, with the exceptions of shoals at channel outlets (KHE 2011a). In 2009, ebb shoals or gravel bars developed at the mouth or downstream of the mouth of the newly constructed channels draining the East Pasture, with accretion during the first year totaling more than 1 to 2 feet (KHE 2009b). These deltaic-type shoals had encroached into the mainstem Lagunitas Creek channel, reducing the cross-sectional flow area, although they did not span the full width of the channel (KHE 2009b). While both of these shoals rapidly formed after restoration, their evolutionary paths have diverged somewhat. The horseshoe-shaped Tomasini Slough outlet shoal has remained relatively consistent in elevation between 2010 and 2011. It is comprised of an inner and outer shoal that range in elevation from 1- to 2 feet NAVD88 (KHE 2011a). Conversely, the shoal at the mouth of the new side channel off Lagunitas Creek has continued to accrete or build in elevation with estimated deposition rates of 0.7 feet in WY 2010 and 0.75 feet in WY 2011 (KHE 2011a). With post-restoration winters being relatively dry, little energy in the way of flood scour has been available to counteract deposition of sediments at the mouth of new tributaries to Lagunitas Creek, therefore leading to a net depositional environment. Should flood flows continue to be reduced, shoals will continue to build in Lagunitas Creek and perhaps change circulation and drainage patterns in the creek and wetland.
While elevations may have increased at the mouth, both of the newly constructed channels have actually deepened since restoration was completed. By 2010, the downstream portions of Tomasini Slough had decreased in elevation relative to constructed elevations by as much as 1.8 feet, with an additional drop of 0.75 feet during the next year (KHE 2011a). The one upstream station with historic data showed little elevation chance since pre-restoration conditions (KHE 2011a). A similar pattern of channel incision occurred at the newly created side channel off Lagunitas Creek in the East Pasture. At least 1 foot of both channel deepening and widening took place in downstream portions of this small tidal creek, while upstream portions widened, but actually became more shallow through deposition of approximately 1 foot of sediment (KHE 2011a). Unconstructed channels are also becoming deeper: these are naturally developing channels on the marsh floodplain. Unfortunately, the lack of vegetation, particularly in the northern portion of the East Pasture, may slow down this process somewhat by encouraging overflow of tidal waters and floodwaters onto the marshplain rather than keeping them in channels (KHE 2010a).
Interestingly, marshplain areas appear to be gaining in elevation in both the East and West Pastures, despite the massive vegetation die-off in the East Pasture that would be expected to compact soils due to loss of root volume below the soil surface (Parsons and Ryan, in prep.). In both pastures, elevation gains exceeded sediment deposition rates measured through use of feldspar markers, with elevation gains between 2008 and 2010 ranging from 13.5 mm in the West Pasture to 19.2 mm in the East Pasture and sediment deposition rates for that period ranging between 0.9 and 5 mm annually in the East and West Pastures (Parsons and Ryan, in prep.). In 2011, the trends in elevation shifted somewhat, with elevation again increasing in the West Pasture (4.5 mm) relative to 2010 elevations, but decreasing in the East Pasture (-10.2 mm), although, overall, elevations were still higher post-restoration than pre-restoration (Parsons and Ryan, in prep.). Interestingly, some of the other sampling sites that had not appeared to gain in elevation since 2008 such as the western end of Walker Creek Marsh in the northern end of Tomales Bay had positive elevation increases for the first time in 2011 (4.5 mm; Parsons and Ryan, in prep.). Most of the elevation gains in the restored wetlands appear to result from changes in subsurface processes, with reintroduction of tides potentially increasing porewater volume in the soils and slowing down subsurface oxidation rates of organic matter (Parsons and Ryan, in prep.).
Most of the sediment deposition occurring in Lagunitas Creek and the Project Area appears to come from re-working of soils from the Project Area, which are now exposed and vulnerable after construction and decay of pasture vegetation. With the first winter being a dry one, sediment inputs from the upper watershed were probably minimal, particularly as there were no overbank flooding events. While the 2009/2010 winter was much wetter, there was still not enough flow volume during storm events to cause overtopping of creek banks in the Project Area, and, thereby, any deposition of sediment on newly restored marshplains. Some overtopping did occur during the winter of Year 3, when rainfall totals were even higher than Year 2, but there was only one very brief event. Even during large storms, most of the peak flood flow and sediment generated are trapped by upstream dams, reducing flood volume and sediment loading to downstream areas.
Despite this lack of overbank flooding, sedimentation monitoring has shown that sediment was still deposited on Project Area marshplains during the last three years (Parsons and Ryan, in prep.). While much of this sediment may derive from re-working of Project Area soils, annual deposition rates since restoration appear similar between the northern portion of the East and West Pastures and the very northern portions of the Undiked Marsh near Tomales Bay, which is unlikely to be greatly affected by sediment re-working within the restored wetlands. Not only have dry winters and reduced flood flow volume affected sediment deposition rates, they also affect stream energy and the ability of the creek to erode newly deposited sediments in and at the mouth of tributaries. In general, this leads to a net depositional environment both within the Project Area and Lagunitas Creek, except where flow velocity is high enough to counteract this trend, such as in the downstream portions of the Tomasini Slough and the Lagunitas Creek side channel. Should flood flows continue to be reduced, shoals such as described above will continue to build in Lagunitas Creek and perhaps change circulation and drainage patterns in the creek and wetland.
Essentially, the Giacomini Wetlands are in the process of hydrologic evolution. The conditions predicted by hydraulic modeling represent a later phase in wetland development. Over the coming years, existing and created channels will continue to increase in size to accommodate flood flows, and new tidal channels will develop, increasing exchange between the restored wetland and Lagunitas Creek and creating more of an equilibrium between tidal inflow and outflow. In addition, some portions of the higher elevation undiked marsh outboard of the levees may continue to erode (as they have been doing prior to restoration), allowing more tidal waters to sheetflow across the marshplain back into Lagunitas Creek. Some of these changes may be accelerated during flood events, although storms so far have not been of sufficient magnitude to dramatically alter the wetland landscape.
This evolution appears to be already well underway. Hydrologic data suggested that the marsh was draining slightly faster during outgoing or ebb flows in 2009 than 2008 (KHE 2010a), and drainage improved slightly again between 2009 and 2011, at least during spring tides (KHE 2011b). Low tide elevations continue to be constrained as they were prior to restoration by the presence of gravel and sand bars at the mouths of creeks, which keep water levels at about 2.0 feet NAVD88 (KHE 2011b).
This improvement in drainage efficiency can be seen in the dramatic declines over the last few years in the extent of subtidal areas during an extreme low tide. Acreage declined from 109.4 acres in the East Pasture immediately after restoration to 68.1 acres in summer 2010 under approximately equivalent tide conditions (-0.44 to -1.74 ft MLLW in 2008 vs. -1.54 to -1.67 ft MLLW in 2010, Figure 2 - 220 KB PDF). This represents a 38% decrease in extent of permanent inundation during extreme low tides within two years. In 2011, this trend continued (Figure 4 - 347 PDF): acreage of subtidal areas in the East Pasture dropped to 51.0 acres, even though water levels may have been influenced somewhat by the unusual rainfall pattern in WY 2011, where precipitation extended well into the summer. (Summer stream discharge flows averaged 10 cfs compared to the median estimate of 6 cfs, which may have kept the marsh from fully draining during low tide events.)
This situation demonstrates that ecosystem evolution following restoration is not a linear process, but can occur in distinct stages or phases that involve triggering or exceeding thresholds before the wetland moves into the next evolutionary stage or phase. This process is described in more detail under How Long Will It Take for Tidal Wetlands to Develop.
Figure 1 - Predicted Tidelines (940 KB PDF)
Figure 2 - Extent of High Tides Post-Restoration (220 KB PDF)
Figure 3 - Change in Tidal Regime in East Pasture's Tomasini Slough (52 KB PDF)
Figure 4 - Change in Extent of Subtidal Areas within the Restored Wetland (347 PDF)
Figure 5 - Change in Salinities and Water Levels at northern portion of Lagunitas Creek in Project Area (407 KB PDF)
Kamman Hydrology & Engineering (KHE) (2006). Hydrologic Feasibility Assessment Report: Giacomini Wetland Restoration Project. Point Reyes National Seashore. Point Reyes Station, California, Prepared for Point Reyes National Seashore.
KHE (2009a). Olema Marsh Restoration: Salinity Impact Assessment. Prepared for Point Reyes National Seashore, Point Reyes Station, California.
KHE (2009b). Evaluation of recent topographic profiles at selected locations in Lagunitas Creek
KHE. 2010. Giacomini Wetlands 2009 Monitoring Data Summary. Prepared for Point Reyes National Seashore Association.
KHE (2011a). Giacomini Wetlands 2009 Monitoring Data Summary. Prepared for Point Reyes National Seashore, Point Reyes Station, California.
KHE (2011b). Survey of Cross-Sectional Profiles in the Giacomini Wetland Restoration Project Area. Prepared for Point Reyes National Seashore, Point Reyes Station, California.
Parsons, L. and A. Ryan. in prep. Keeping Pace with the Tides: Results of SET and Marker Horizon Monitoring -- Giacomini Wetland Restoration Project, Tomales Bay. Point Reyes National Seashore.
Did You Know?
A 1-foot sea level rise can lead to shorelines eroding back 100 feet, and increase the chances of a 100-year flood event in low coastal areas to once every 10 years. More...