Characterizing hydrologic settings and hydrologic regimes of headwater riparian wetlands in the Ridge and Valley of Pennsylvania
Open Access
- Author:
- Hychka, Kristen Carol
- Graduate Program:
- Geography
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 01, 2010
- Committee Members:
- Robert Brooks, Dissertation Advisor/Co-Advisor
Robert P Brooks, Committee Chair/Co-Chair
Alan H Taylor, Committee Member
Douglas Alan Miller, Committee Member
Christopher J Duffy, Committee Member - Keywords:
- wetlands
landscape setting
reach scale
hydrologic regime
headwaters - Abstract:
- Riverine systems are highly complex and dynamic systems. They have been described as having a four-dimensional nature of fluxes of materials, energy, and biota in a riverine system through: lateral connectivity between the channel and the riparian area or floodplain; longitudinal connectivity between channel segments upstream and downstream; vertical connectivity between the channel or floodplain and the aquifer; and temporal dynamics of the system. Floodplains can be seen as the ‘beads on a string’ of the riparian corridor where maximum three-dimensional hydrologic exchange allows for the fluxes of energy, materials, and biota within and between heterogeneous habitats. Building on Landscape Ecology and Fluvial Geomorphology a River Ecosystem Synthesis (RES) Framework looks to understand rivers as dynamic shifting mosaics of geomorphic patches or Functional Process Zones. Wetlands are a key part of this shifting mosaic, but have been under characterized and underrepresented in the RES, primarily due to a lack of cross-pollination between wetland and stream ecology. This research intended to help nudge the discussion of wetlands towards the RES Framework and help include wetlands in the integrated approach being pursued by stream ecologists under the RES. Additionally, there is a general need to better understand the hydrologic setting and dynamics of headwater wetlands. Drawing upon the opportunities to inform and be informed by the stream ecology literature by looking at wetlands in a 4-dimensional, dynamic, River Ecosystem Synthesis approach, this study sought to: better understand the hydrologic setting and the seasonal dynamics of the hydrologic regime for headwater wetlands in the Susquehanna River Basin. The objectives were achieved through a synthesis at the basin scale (Chapter 2) and detailed analyses in a single sub-watershed (Chapter 3) and a subset of wetlands with long-term water level records (Chapter 4). In Chapter 2, this work used the framework of the four-dimensional nature of the RES to generate a revised approach to scaling for the study of wetland services in the Susquehanna River Basin. One aspect of the revised scaling hierarchy was the use of a reach-scale. Chapter 3 used the understanding that in river ecosystems wetlands and, in turn high bio-complexity, occurs where there are laterally and longitudinally unconstrained reaches. This research identified what topographic characteristics defined unconstrained reaches in this physiographic setting based on the known occurrence of wetlands in the study area. Chapter 4 explored the vertical and temporal dimensions of wetlands in this physiographic setting by exploring the relationships between water level, wetland type, and seasonal fluctuations across years with a range of drought and deluge conditions using time series analysis. Chapter 2 is a synthesis of the efforts of the ecological portion of a study of climate change and ecosystem services provided by freshwater wetlands of the Susquehanna River Basin study to characterize and map a hierarchical landscape classification for use in the study. The scaling hierarchy analysis not only identified a gap in spatial scale of data between disciplines, but it identified the reach as a scale to bridge that gap. Building upon several existing classification schemes, a revised hierarchical landscape classification was generated: Basin, Physiographic Province, Sub-watershed, Channel Reach, and Habitat (macro- and micro-). This work not only proposed the use of a reach scale, rare in wetland studies but very common in stream studies, but articulated a process-based macro- and micro-habitat classification. Chapter 3 improved the spatial prediction of headwater riparian wetlands through identification of reach settings unconstrained latitudinally and longitudinally that allow for this three dimensional exchange of water. Known locations of mapped, non-open water National Wetlands Inventory (NWI) wetlands, field-identified non-NWI wetlands, and non-wetland locations (n=40, 30, and 35, respectively) were used to build a predictive partition tree. Predictive variables were DEM-derived topographic indices for the stream reaches: valley width, mean stream slope, and contributing area. The partition tree resulted in a 5-node tree (overall R2=0.61). These classes ranged from very high likelihood of wetland occurrence to very low likelihood of wetland occurrence or least constrained to most constrained. This classification is a useful approach to characterizing wetland and non-wetland reach settings, especially in screening out the least likely wetland-supporting or most constrained reaches within a watershed. Chapter 4 used a suite of time series analyses to explore the hydrographs of five headwater wetlands in terms of their dynamics and response to climatic drivers. Cross correlations between daily differences in water levels and precipitation showed significant correlations for most wetlands under dry and wet conditions on the same day time lag. Of the wetlands evaluated, all experienced a summer drawdown in water level except for the wettest sites in the wettest years. Further, the timing of the beginning of summer drawdown varied greatly for the period of record for the three studied wetlands (slope=61 days; headwater floodplain=58 days; and riparian depression=91 days) with the slope wetland drawing down earlier on average than headwater floodplain or slope wetlands (average day of the year 132, 156, 152 respectively). The moving averages of the water levels generally followed the trends of the downstream stream baseflow, except for the wettest site in the wettest year. Though the hydrologic data are only available as a highly discontinuous record over a 10-year period, more continuous records when analyzed as case studies with time series analyses can give insight into the dynamics and responses of hydrologic behavior of headwater wetlands to climatic drivers. This work is one step in a process to integrate scientific understanding and management of wetlands, floodplains, and streams as one inter-connected system. The River Ecosystem Synthesis Framework encourages this multi-system approach to understanding the 4-dimiensional nature of river ecosystems through the integration of principles and approaches of fluvial geomorphology and landscape ecology into stream ecology. However, the nature and functioning of wetlands in the river ecosystem needs to be incorporated into and advanced by this framework. The work in this study helps to move this integration forward by exploring the four-dimensional nature of headwater wetlands in the river ecosystem, which can contribute to this critical, but limited literature at the nexus between streams and wetlands.