Introduction
The vital importance of the functions of wetlands regarding water treatment and storm surge mitigation has become clear in the last couple of decades. As climate change continues to have tangible impacts, wetlands that have already been decimated through anthropogenic activities are under existential threat with rising sea levels. Identifying the wetlands under greatest threat to target conservation efforts is imperative.
The Lower Hudson River Estuary is a highly modified waterway with a large human population and vastly urbanized areas. Sea level rise, a result of climate change, can potentially eliminate tidal wetlands that are already under stress from historical anthropogenic modifications and their subsequent effects. The primary way these wetlands can naturally offset this is by their ability to deposit sediment, raising their surface elevation over time. Wetlands can deposit sediment at consistent rates, given a regular influx of sediment occurs. Their future resiliency to climate change may reside in matching sediment accretion with sea level rise.
The aim of this report is to assess the status of the tidal wetlands in the Hudson River Estuary regarding sediment accretion and sea level rise. Several studies have been undertaken on this topic on sections of the estuary, but it is essential to get a broader perspective for the overall health of the entire estuary to highlight areas that may have been neglected or that are in more rapid deterioration than others.
Additionally, individual researchers may have approached this subject differently, whether that is by geographical locations, data gathering methods, choice of analysis and computational models, or interpretation of results. Having a baseline of applied methods and analysis would be beneficial in having greater confidence in these findings.
Getting a general sense of the current state of the Hudson River Estuary tidal wetlands, and how imperiled they may be, would allow for proper prioritization. Robustness to sea level rise and any intervention methods that might help and how effective they would be, is the ultimately the goal of this report.
The scope of the study
The geographical area under consideration is the Hudson River from Troy, NY, to New York City, continuing out along the coastal waters to Jamaica Bay. On the New Jersey side of the river, this will include the Hackensack River, Passaic River, Newark Bay, Arthur Kill and Raritan River.
The likely global mean sea level rise by 2100 is 12-24 inches under the low greenhouse gases emissions scenario, 17-30 inches under the intermediate greenhouse gases emissions scenario, and 24-39 inches under the very high greenhouse gases emissions scenario (IPCC, 2021). But sea level rise has been above the global average in the northeastern United States. For instance, sea level rise in New Jersey has been 2.5 times the global average (Kopp et al, 2019). Sea level rise has been higher on New York coasts as well, related to subsidence of the southern shores of Long Island since the last glacial retreat. The following is a table of projected sea level rise for the state of New York in Low, Medium, and High-sea level rise projections:
Even the most conservative estimates predict a 15-inch increase, which could certainly drown some lower lying marshes.
While this may challenge the healthiest wetlands, many marshes aren’t in pristine, or even good, conditions. Some marshes have top heavy vegetation due to a lack of mineral sediments that are needed to strengthen root systems. Some are already being challenged by drowning from subsidence and compaction, water channeling, changing hydrology that create fragmentations and intertidal mud flats (Messaros 2012), erosion, and geese grazing. Definitive loss of wetlands is still happening from encroaching urbanization and development. The following is a graphical representation of wetlands losses in the New York and New Jersey Hudson River Estuary within the prior decade (2009-2017):
New Jersey has had greater losses of wetlands recently because it has had more wetlands to lose. Additionally, it experiences the loss by a thousand cuts; little pieces here and there have added up, due to weaker state restrictions on smaller patches of wetlands near encroaching development (Protecting the pathways, 2016).
The current state of tidal wetlands of the Lower Hudson River Estuary
New Jersey: Lower Passaic, Raritan Bay, Newark Bay/Meadowlands
The Meadowlands, which is situated between the Passaic and Hackensack Rivers before they both empty in Newark Bay, had vast wetlands of 20,000+ acres in the 19th century, but only 8,400 were left by 2019, with 3544 under conservation regulations (Weis et al 2021). Of the seven monitored sites for sea level rise in the Meadowlands, only two sites were keeping pace (Weis et al 2021). Those two were also the sites dominated by Phragmites australis, a species not native to New York or New Jersey.
The Lower Passaic River, heavily dredged with channels that have not been in use for decades, doesn’t have many wetlands to speak of (Mathew et al, 2020). It has relatively few sub-tidal shallows or tidal wetlands. There isn’t much data on this section in general, but there is some sediment analysis, showing that it is a net sink for sediments during low river flow condition (Mathew et al, 2020).
Raritan Bay has shown no changes to wetland acreage and composition in the last 40 years but that is most likely due to the lack of published data; therefore, this area needs additional studying and data collection (Weis et al 2021). None of the 4 monitored sites there were keeping pace with sea level rise (Weis et al 2021). An important caveat is that the data for Raritan Bay was gathered not by using Surface Elevation Tables (SETs), but sediment plates, which are susceptible to disturbance. But SETs were recently installed by Rutgers University, with data results forthcoming soon (Weis et al 2021).
New York: Upper Estuary, Mid-Hudson, New York City
Tabak et al 2016 looked at sediment accretion and sea level rise starting with the Lower Patroon and Breaker Islands (river mile 145), south to Piermont Marsh (river mile 25). As of 2007, this stretch of the Hudson River had approximately 6,900 acres of tidal wetlands. In projected sea level rise scenarios by the computer modeling program SLAMM (Sea-Level Affecting Marshes Model), total wetland area shows increases, from 1,136-3,953 acres, due to wetland migrations. Yet the increase isn’t as great as it could be because the projection shows some of the current lower wetlands lost to open waters. Wetlands in these areas seem to have generally high survival rate due to migration upland and/or matching accretion rates with sea level rise, if sea level rise is not in the highest level of the predicted range. But the composition of the wetlands change, with more highland marshes becoming lowland marshes, and lowland marshes become inundated. Below is the graphical representation of those results:
Tabak et al also try to classify the wetlands by the degree of resiliency: Persists as is, changes to another class of wetlands or creation of new wetlands by freshly inundated uplands, complete loss, and new wetlands that could run into impediments to upland migration. If sea level rise doesn’t go up dramatically, losses may be less than 15%, which would be offset by creations of new marshes. But composition could change significantly if accretion rates don’t keep pace, which means a gain or loss of specific habitats, creating an impact on residential species and uncertain ecological effects.
Yellen et al 2020 emphasizes that there is limited area for wetland migration in some parts of the Hudson River estuary, namely around the Highlands, because the wetlands abut steep rock formations where bedrock was scoured by water and glaciers (Yellen et al 2020; Tabak et al 2016). Yellen et al looked at sediment accumulation at five wetland sites from Stockport NY (river mile 120) to Iona Marsh (river mile 46). Compared to Iona Marsh (the undisturbed reference site), the other 5 marshes (Tivoli Bays, Stockport, Esopus, Vanderburgh) have been anthropogenically impacted in that they were physically altered in some way in the last century, yet they have accumulated sediments at much quicker rates (10-30 times) than Iona Marsh, the reference site. And more than half of those newer marshes have grown in last 120 years.
Clough et al 2016 analyzed shorelines from the Tappan Zee Bridge in New York City to Long Island using SLAMM. This area contains about 30,000 acres of marsh. Their conclusion was that high marshes are the most vulnerable, as with sea level rise high marshes become low marshes. Predicted losses range from 8,600 to 20,000 acres. Predicted gains from of low marshes converting from high marshes range from 5,400 acres to 10,300-17,500 acres by 2100. This includes some inevitable loss of low marshes converted to open water. Therefore, even in the worst scenarios, total marsh area could stay approximately the same if land use stays as is with no future development, to allow for wetland migration.
Peteet et al 2018 analyzed Jamaica Bay using sediment cores. 6 wastewater treatment plants have their effluents empty into Jamaica Bay, which is this bay’s primary hydrological input. The contents of the effluents used to act as nutrient inputs which helped the sediment accretion rate for Jamaica Bay. But now that the NYC wastewater is having nitrogen removed before releasing the treated water per efforts at cleaner effluents, this nutrient source is no longer available to the wetland vegetation, and the accretion rate may not keep pace with sea level rise. Additionally, an important distinction should be made between organic and mineral matter in sediments. Wastewater is primarily organic material. Jamaica Bay had historically decreased levels of mineral inputs, because very little runoff came from groundwater as it got diverted for human use. The organic material without corresponding mineral inputs makes the vegetation top-heavy and creates weak roots. Cahoon et al 2019 also focused on wetlands restoration in Jamaica Bay. They compared an un-restored control area, and area restored with a thin layer of dredged sediment. The restored marshes are faring better than unrestored, and some unrestored wetlands are already in danger of drowning. Chant et al 2020 showed that sediment in Jamaica Bay comes mostly from Hudson tidal movement during storms and spring tides, but it doesn’t seem to be getting to the marshes where it is needed most, because of previous channeling. Due to the hydrology and weakening of tidal movement in bay, sediments from estuary may be preferentially depositing into the deeper main channels instead of aiding in wetland accretion. Therefore, this other supply of sediment into the bay isn’t enough to help keep up with rising sea levels.
Overall, wetlands loss has slowed significantly over the last few decades due to tightening regulations but needs to come to a full stop to preserve what is left. The general health status seems to be somewhat robust yet needs active management for these newer climate change related threats. Some places in estuary also seem to be missing data (Raritan Bay, Lower Passaic River), and that would be needed before making a confident assessment. The overall picture is brighter than feared, but the only sure way to succeed in preservation is active data collecting and management.
Research methods
Reasons why wetlands fail vary, so individual site assessment is important. Not only is characterization of the wetlands vital (fresh vs tidal, low vs high marsh, type of vegetation, etc.), but specific threats are too. Dredging and channeling change hydrology, and can cause erosion, inner fragmentation, sediment changes, rising water levels, root system collapse, invasive species, subsidence, eutrophication, toxin inputs, and overgrazing (Messaros et al, 2012). But how they are characterized is also important for comparing across studies and assessing outcomes. All the studies mentioned above used a variety of methods, listed below:
- SETs: These Surface Elevation Tables, which are non-destructive, and can measure change in sediment levels over time (Lynch et al 2015). They have been used in the Meadowlands and Raritan Bay (Weis et al 2021), Jamaica Bay (Cahoon 2019), and at several sites further up the Hudson River (The State of the Hudson, 2020; Raposa et al, 2016). Below is an example of data from a SET:
- Sediment Cores are usually taken for creating a historical baseline analysis, and have been taken at Jamaica Bay, Lower Passaic River and further up the Hudson River (Yellen et al, 2020; Peteet et al 2018; Mathew & Winterwerp, 2020). This is a somewhat destructive process as the soil is taken away from the site but is usually of minimum impact considering the size of wetlands in general. And it doesn’t need to be repeated once an adequate historical baseline has been created.
- SLAMM (Sea-Level Affecting Marshes Model) (Clough 2016; Tabak et al, 2016) is a computer model that tries to predict the likelihood of survival of marshes during sea level rise. But in the two studies above that used this program, their results are not comparable because these two studies didn’t use same parameters, even though the study sites overlapped (Tabak et al, 2016).
- Moored sea instruments and shipboard surveys were used at Jamaica Bay (Chant et al 2020), and the Lower Passaic River (Mathew & Winterwerp, 2020). Bathymetry data was also used at the Lower Passaic River (Mathew & Winterwerp, 2020).
- COAWST (Coupled Ocean Atmosphere Wave Sediment Transport Modeling System) was used in Jamaica Bay, which attempts to assess sediment transport (Chant et al 2020).
- HydroQual Model was used on the area from the Lower Passaic River to Staten Island, to assess currents and bed sheer, although this seems to be a method more useful for sediment analysis than wetlands sediment accretion analysis (Mathew & Winterwerp, 2020).
These are just some of the methods that are used in sea level rise analysis. Most studies only used a couple of these methods in any single research regimen. The differing data sources and analyses makes it difficult to compare the outcome of one study to another to see if there is any sort of agreement among the results.
Mitigation efforts: migration pathways, vegetation, dredged sediment and living shorelines
Migration pathways for wetlands are when there is space and gentle enough slope just upland of the wetland, to allow it to move up in elevation (Weis et al, 2021). The high marsh gets inundated and becomes low marsh, and then the high marsh moves higher. This can be helped by removing any raised embankments, adjusting slopes, and preventing development (Weis et al 2021). Clough et al 2016 suggests using road data to figure out which roads may be flooded from rising sea levels, and which may impede marsh migration for mitigation efforts. The range of sea level rise scenarios should also be taken into consideration. Impediments to this approach are having to work with private landowners, although preserving these spaces decreases flood risks to owners and the surrounding communities (Protecting the Pathways, 2016).
As tidal wetlands migrate is desired, some of those pathways are currently unprotected and could become developed before wetlands have a chance to move in. The tidal wetlands on the Hudson River that are closer to Troy have the largest land masses that could be conserved for wetland migration. It is unfeasible for every wetland to be saved, so efforts should be prioritized in places like these, especially on public property. For private property owners, conservation easements can be made more enticing while also keeping in mind that rising sea levels could soon change which lands need to be conserved, to secure protection for the long-term future (Protecting the pathways, 2016).
Almost half of current wetlands and their possible migration paths are on public or protected property. Additional potential sites to take advantage of are land masses inundated by water, and those that were created by depositing dredges soils that aren’t explicitly protected and should be – this accounts for almost 20% of current tidal wetlands on the Hudson River north of New York City. These lands are owned by the Office of General Services, and efforts have been underway to have these transferred to one of the conservation agencies of NY state (Protecting the pathways, 2016).
Below are two maps of the Hudson River with different classifications of tidal wetlands in the estuary. According to the maps, there seems to be significant potential upland available for wetlands migrations; it just needs to be conserved and to remain undeveloped. This first map is the Hudson River north of Westchester County, and the second map is New York City:
Management of different kinds of vegetation can be an important aspect of wetlands restorations. Phragmites australis (Weis et al, 2021) is an invasive version of a native plant that has more biomass and nitrogen and carbon storage potential that the native species Spartina alterniflora, and creates more rapid increases in marsh elevation (Davidson et al 2018). P. australis also traps sediment and forms dense roots (Rooth and Stevenson 2000). P. australis was originally being aggressively removed because it has been shown to decrease local biodiversity (Chambers, et al 1999), but now it seems like leaving it in some places might be a better option; inferior marsh still seems better than no marsh at all, given the choice. Yet, Yellen et al found that anthropogenically impacted marshes on the Hudson River have heavily favored production of cattail (Typha spp) over Phragmites, with two-thirds of cattail marshes occurring in previously disturbed marshes. Clearly, native species are still outcompeting P. australis in some conditions.
In Tivoli and Stockport, small initial invasions of P. australis were contained. By managing and controlling P. australis at these two locations, “nearly 1,030 acres of native marsh has been protected or restored” (The State of the Hudson, 2020). In wetlands that are in good health with a high likelihood of survival, eliminating Phragmites might be a worthwhile endeavor, but in marshes in poor health or greater risk of drowning, leaving Phragmites might be a better option. At Iona marsh, about 42 acres of marsh that was dominated by P. australis was changed back to native vegetation (The State of the Hudson, 2020) https://nystateparks.blog/2018/11/13/marsh-madness-restoration-of-iona-marsh-from-invasive-phragmites/While Iona Marsh is considered an unaltered reference site deserving of preservation efforts, the time and money that it takes to remove a hardy well-established invasive species that might ultimately help the survival of the wetland should be heavily considered.
Sediment manipulation, a term used by Weis et al 2021, is the repurposing of dredged sediment to build wetland ‘elevation capital’, done by putting a thin layer of sediment on or near the wetlands for the wetlands to assimilate. Care must be taken not to smother or kill biota, or not to place it where it just washes away (Weis et al 2021). This method has been applied with varying success on small scales in several spots in New Jersey (Weis et al 2021). It has also been done with varying success in Jamaica bay (Peteet et al, 2018), but it is challenging at this site as the previously dredged areas are where some sediments settle. The composition of repurposed sediment may be important depending on the deficiencies of the wetland that is attempting to be restored or helped against sea level rise. The Army Corp of Engineers has been trying to create and restore wetlands with dredged materials for several decades, but has been specifically working in Jamaica Bay on these types of projects (Messaros et al, 2012). In one example, dredged sediments were placed on five of the small marsh islands in Jamaica Bay. The composition of the dredged material was primarily sand, and once placed, it was planted with native vegetation. At this time, according to the USACE, approximately 155 acres of wetlands in Jamaica Bay have been restored in this manner.
Living shorelines use natural materials to replace, support, and maintain the shoreline, enabling wetlands migration and flood mitigation. It has generally been successful in meeting erosion goals, but its fate is still undecided for maintaining and aiding wetlands health. This is mostly because historically monitoring post-construction is 1-2 years, and wetlands need longer monitoring to see results (Weis et al 2021). A similar approach is the use of polders in Europe, which are little walls of brushwood that look like stick bundles; This seems to help if there is already a large sediment load in the waters, where a physical yet permeable barrier keeping some sediment from reaching the wetlands is not a hinderance to accretion (Yellen 2020).
The Hudson River Natural Estuarine Research Reserve has a program called The Hudson River Sustainable Shorelines project: https://hrnerr.org/sustainable-shorelines/ This program partners with government agencies and local communities to create living shorelines. To date, they have already improved 14 sites along the Hudson River Estuary. The following is a map of living shorelines created in partnership with the Nation Oceanic and Atmospheric Administration (NOAA):
Hudson River Estuary Action Agenda (HREAA) aims to conserve 375 new acres of wetland migration pathway by 2025 and 750 acres in total by 2030, with the intention of focusing on migration pathways regarding sea level rise due to climate change. They have already restored 103 acres of shallow/intertidal areas and aim to improve 20 acres more by 2025 and 30 in total by 2030. They also intend to use SETs to assess sediment accretion, but at this time it seems unclear where, or how many will be implemented. HREAA is targeting land acquisition for areas adjacent to Hudson River based on migration pathways, and they are in the process of identifying the best management practices for sustainable shorelines to be considered in areas that have proposed development.
Recommendations:
An estuary-wide mapping of wetlands for greater resolution is key, as wetland losses may be overestimated or even underestimated in some areas. All wetlands are different and have different risks. More active managements on smaller scales to assess these risks is necessary for successful risk mitigation measures for future inundations. In addition to restoration of current wetlands, and conserving uplands for migration, perhaps other shorelines can be explored to create more tidal wetlands. Previously disturbed areas that now have thriving marshes show that wetlands can come back given the opportunity, especially in areas that show sediment movement and slope would support them. The Hudson River Park just created one on a pier in NYC: “The Tide Deck, located at the western edge of Pier 26, is an engineered rocky salt marsh created to provide an immersive and educational river ecology experience for Park patrons—not to mention a supportive environment for wildlife.”
Additionally, Cornell University has a program called The Climate-adaptive Design (CaD) Studio with the NYSDEC Hudson River Estuary Program. “The CaD Studio links Cornell University graduate and undergraduate students in landscape architecture with high flood-risk Hudson Riverfront communities to explore design alternatives for more climate-resilient and connected waterfront areas.” Some past projects have explicitly attempted to create and implement wetland designs for Catskill, NY, Ithaca, NY, and Hudson, NY.
There seems to be a need for a more ubiquitous baseline assessment of the whole estuary. SETs for current data and sediment cores for historical data could be implemented to get a past and present picture. From there, an estuary-wide analysis with one of the computer modeling programs would be helpful to get a sense of sea level rise risks as a whole; comparison to earlier results in smaller areas could help assess over or underestimations or possible variances in data analysis. The adaptation of some if not all the MARS Index (Raposa et al, 2016) for data gathering would be ideal. The MARS Index (MArsh Resilience to Sea-level rise) developed by the National Estuarine Research Reserves, are the use of five measurements over time that inform 10 data metrics for better resolution of wetland resilience pertaining to sea level rise. It uses sediment cores, SETs, Tidal ranges, Sea-level rises, turbidity (or suspended sediment), and marsh elevation distribution (ex. how much of marsh is below the mean high-water mark). If these were the standards used, they could all be compared within the watershed.
In general, the health of the tidal wetlands in the Hudson River is better than expected. While the tragic historical losses of wetlands in the area cannot be gained back, current losses have slowed considerably as restrictions and regulations tighten. Several non-profit and governmental agencies are working diligently to preserve and improve the size and status of these wetlands. And while sea level rise is very concerning, there remains very clear ways to aide in the preservation of what we have left. It is doable, we have the tools and methods, and with enough forethought and effort, there will still be approximately the same acreage or greater, of wetlands in 100 years as there is today.
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