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Friday, July 23, 2021

The wetlands’ oldest stories are told by silt, sediment

Photo courtesy Darryl Saffer
A shallow wetland pool in the Riverwalk conservation area.

By Josh Pike
Pagosa Wetland Partners

Wetlands tell a myriad of stories. Stories in the flitting of birds as they feed and nest. Stories in the patterns of vegetation growth and the channels of water that wind through them. 

The oldest stories in the wetlands are told by the silt and sediment that accumulates at the wetland bottom. These soils chronicle the history of the wetlands, telling us stories of droughts and floods, of changing water sources and vegetation and, stories of human uses of the wetlands and the changes they have wrought.

The field involved in reading wetland soils is a geology field called stratigraphy. Stratigraphy involves interpreting the position and relationship of various layers of soil (strata) in the sediment. The interpretation process begins with the collection of a sediment core, a tube-shaped sample of sediment taken by drilling into the ground and extracting a vertical cross section of the accumulated soil in the area. This core is then painstakingly analyzed, with the composition of the various strata determined and the relationship between adjacent strata analyzed. 

Wetlands are particularly suitable places for this type of analysis due to their saturated soils. In most areas, fine objects, such as pollen grains, quickly decay and are invisible to later stratigraphers. However, the water-saturated soils of wetlands slow decomposition by reducing the amount of ambient oxygen available for decomposition. This slowed decomposition rate preserves many objects, like phytoplankton shells, that would otherwise quickly decay and pass out of the record. 

Additionally, because wetlands typically collect material from surrounding areas through the water draining into them, the sediments in wetlands offer an excellent perspective on conditions in the general area near the wetlands.

The stories told by the wetland sediments can be divided into two main categories, stories about changing climate and landscape and stories about the organisms that grew within and around the wetlands. 

Stories about landscape and climate change are recorded in the width and composition of the bands of sediment in the wetlands. These bands of sediment serve as indicators of soil and climate conditions in the wetland catchment (the area of land which drains water into the wetlands). If the surrounding soils are eroding rapidly, the sediment layer left behind will be thick and coarse as a result of the large amount of relatively rough material that was deposited. Such rapid erosion can be caused by several factors. These include deforestation or a similar natural or human-caused removals of vegetation which reduce the stability of the soil and increases erosion. 

Alternatively, increased erosion can be caused by increased precipitation. Fine muddy sediments can be indicators of extensive vegetation growth as only fine dirt particles move through the vegetation and end up in the ponds.

Grains of charcoal within the sediment serve as indicators of nearby wildfires or human burns. The proximity of these fires to the wetlands can be determined by the size of the charcoal fragments and their source can be determined by context, with human fires often being followed by extreme and abrupt changes in the use of the surrounding landscape which does not typically occur with natural fires.

Stories about the types of organisms surrounding the wetlands are recorded in fossilized fragments of plants, animals and pollen buried in the sediments. Grains of pollen and the shells of diatoms (a group of microscopic algae) are particularly helpful indicators of the inhabitants of the wetlands and the surrounding area. These remains can often be compared with current species to determine what general types of organisms or specific species lived in the area. 

Such identifications have their limitations, particularly in the case of pollen, as pollen can be carried significant distances through air or water and are not necessarily accurate indicators of the species composition of the specific wetland in the past. However, pollen remnants are excellent indicators of the composition of species in the region. 

Additionally, the wetlands can preserve the remains of human tools and weapons and even corpses. These include the famous bog bodies, Iron Age corpses found buried in bogs throughout northern Europe. These artifacts can provide unique insights into the human inhabitants of the area, complementing the records of landscape conditions and nearby organisms that the wetland soil harbors.

The sedimentary story of the local wetlands found in the Riverwalk Conservation Area is quite young. Restored to their current form approximately 20 years ago after extensive gravel excavations, the first sedimentary pages are just being laid in a story that could last centuries or millennia. However, as long as the wetlands exist, the sediments will continue to accumulate, chronicling the climate changes, the births and deaths of animals, and the human developments that occur around them. In the future, even after the history of Pagosa Springs and its current inhabitants are long forgotten, a natural history, held in the mud at the bottom of a wetland pool, will remain for future scholars. 

If you want to learn more about Riverwalk Wetlands or get involved in helping preserve them for future stratigraphers and archaeologists, please contact Pagosa Wetland Partners at pagosawetlands@gmail.com.

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