Breath of Clarity

Comment #1: Hydrology

Original Post by Hannah Claycomb:

1. What is the geologic history of the US that gave this continent so many wetlands? Do you think that the eastern seaboard of the US which is a passive margin attributes to the huge coastal wetland complex there? Why?

Historically, there are many events that have caused the mass amount of wetlands to form across the United States. Around 9,000-12,000 years ago, when the last ice age was coming to an end, many wetlands were formed in the northern states (NPS 2016). Glaciers dammed rivers, scoured valleys, and reworked floodplains, leaving wetlands to form. Blocks of ice would be left behind by receding glaciers, forming pits and depressions in the land, later filling up with water that would be deemed a wetland. Rising sea levels also tend to create wetlands, flooding coastal lowlands.

Stable, slow rates of rising sea levels, calm tectonic activity, and sediment produced wetlands on the east coast (Braswell, Heffernan adn Kirwan 2020). The huge coastal wetland complex along the east coast was allowed to form due to passive margins and minor seismic interruptions. According to one study, scientists discovered sea level rise was the number one indicator of east coast wetland formation (Braswell, Heffernan and Kirwan 2020). After sea level rise slowed after the ice age, wetlands started to form, thus, wetlands are unable to form or persist under periods of great sea level change. If the east coast were to be an active margin with high tectonic activity (much like the west coast), and therefore subject to greater sea level change, the wetlands would be vastly different.

2. What is the hydroperiod of the wetland you have observed?

“Hydrology is probably the single most important determinant of the establishment and maintenance of specific types of wetlands and wetland processes (Mitsch and Gosselink 2015, 112)”. The hydroperiod of a wetland entails its inflows and outflows of water, basin geomorphology, and subsurface conditions (Mitsch and Gosselink 2015, 111). Hydrology of a wetland heavily influences its physiochemical environment. Therefore, hydrology affects biota, abiotic factors, nutrient availability, and sediments of a wetland.

The wetland I have observed is located in western Pennsylvania. This particular wetland was adopted and constructed by the Allegheny Land Trust after decades of contamination from mining. The creek that flows nearby was rerouted 70 years ago to make way for mining operations. The damage it caused effected habitats and natural processes like sedimentation and nutrient and oxygen gradients. This freshwater wetland is considered a bur reed marsh located in proximity with Chartiers Creek and a riparian forest. The hydroperiod of this wetland mimics lower levels of saturation in the growing season due to evapotranspiration, and higher levels of saturation during winter and spring due to snow melt and seasonal precipitation. Flooding pulses also occur when heavy rainfall and storms transpire during the summer months.

3. I worked in a cypress-hardwood, forested wetland where the Army Corps of Engineers wanted to dam to make permanent, standing water for recreation. With your knowledge of the hydroperiod and primary production – why did we stop the project?

When a dam is built, water becomes stagnant and this decreases productivity in wetlands (Mitsch and Gosselink 2015, 146). Stagnant water caused by dams lowers species richness, effect chemical and sediment characteristics, destroy habitats, deprive wetlands of nutrients, and increase the effects of climate change. Vegetation in stagnant water decomposes and releases carbon dioxide and methane, worsening climate change. Further, after constructing a dam, sedimentation builds up and deprives downstream waters of necessary sediments and can lead to erosion. Wetlands with flowing water or pulsing hydroperiods contain nutrients that are cycled rapidly. When dams are built, nutrient cycling slows and greatly effects the health of the wetland.

When establishing a dam near a forested wetland, growth is ultimately hindered. Highest productivity in forested wetlands occur when systems are not too wet and not too dry that have average hydrologic periods or seasonal hydrologic pulsing (Mitsch and Gosselink 2015, 146). If a dam was built in a forested wetland, increased stresses caused by an anaerobic root zone would occur and it would decrease the length of the growing season (Mitsch and Gosselink 2015, 148).

Braswell, Anna, James Heffernan, and Matthew Kirwan. 2020. “How Old Are Marshes on the East Coast, USA? Complex Patterns in Wetland Age Within and Among Regions.” Geophysical Research Letters.

Mitsch, William J. and James G. Gosselink. 2015. Wetlands. 5th ed: John Wiley and Sons, Inc.

National Park Service. 2016. “How are Wetlands Formed?”,and%20depressions%20in%20the%20land.

My Comment:

Hi Hannah (Question 1),

Great specificity in the info about the impact of sea level rise on wetlands. It is interesting to note that slow rates of sea level rise attribute to wetland formation, and the decreased rate of sea level rise that occurred after the last ice age also attributed to the wetland creation. It aligns with the stable quality of the passive margin attributing to the huge coastal wetland complex at the eastern seaboard because a massive surge in sea levels would not necessarily be conducive to forming wetlands.

From there, I dived into the source from Geophysical Research Letters. The researchers mentioned that their data supported previous work that already showed tidal marshes are unable to form and persist under periods of great sea-level change because it limits adequate sedimentation and macrophyte survival (Braswell, Heffernan adn Kirwan 2020). Also, I found it intriguing that age distributions varied as a function of upland or riverine influence that the researchers measured by RTR (ratio of annual river discharge to tidal estuary volume). Areas with higher RTR should have greater sediment delivery, build elevation faster, and therefore, those areas are more likely to have a date that corresponds with the deceleration of sea level rise (Braswell, Heffernan adn Kirwan 2020). Additionally, the researchers drew a connection that younger wetlands in the northeast and southeast correspond to European colonization and deforestation (Braswell, Heffernan adn Kirwan 2020). The paper clearly illustrated determinants of wetland creation.


Braswell, Anna, James Heffernan, and Matthew Kirwan. 2020. “How Old Are Marshes on the East Coast, USA? Complex Patterns in Wetland Age Within and Among Regions.” Geophysical Research Letters.