Graduation Year

2023

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

School of Geosciences

Major Professor

Mark C. Rains, Ph.D.

Committee Member

Patricia Spellman, Ph.D.

Committee Member

Kai C. Rains, Ph.D.

Committee Member

Matthew J. Cohen, Ph.D.

Committee Member

Joshua H. Viers, Ph.D.

Keywords

Clean Water Act, hydrologic connectivity, intermittent streams, topology, variable source area, vernal pools

Abstract

Headwater wetlands are connected to one another and to downstream waters by dynamic hydrologic flowpaths, functioning as integrated hydrologic networks at the watershed scale. Headwater wetlands perform a variety of hydrologic lag, sink, and source functions, including flow generation and propagation, thereby contributing to the natural flow regimes of downgradient waters. The functions of individual wetlands and their contributions to hydrologic connectivity and subsequently to the natural flow regime have been widely studied and are well understood. Comparatively, the functions and hydrologic connectivity within wetland complexes as a whole and their collective subsequent contributions to the natural flow regime of downstream waters are poorly understood. Additionally, it is generally accepted that wetlands in riparian and floodplain regions are physically, chemically, and biologically integrated with stream and river networks. However, the presence of hydrologic connectivity within certain classes of wetland systems with non-continuous or non-readily apparent surface water connections, including in complexes of certain types of headwater wetlands, has yet to be established.

Headwater hydrologic features such as wetlands, swales, and high-order streams remain poorly understood in terms of their cumulative functionality, and their connectivity to downstream waters remains to be established and recognized. This work aimed to investigate how the hydrologic connectivity of headwater wetland systems as a whole contributes to the natural flow regime of their resident hydrologic systems, and subsequently to the integrity of downstream waters.

The first study in this work investigated the hydrologic connectivity of vernal pool depressional headwater wetland features to the downstream waters of the Sacramento River in the northern Central Valley of California. This study demonstrated that vernal pool wetland features are sources of streamflow and are hydrologically connected downgradient, indicating that they play an important role in maintaining the integrity of downstream waters.

The second study in this work created an archetypal model that can be used to simulate and analyze cumulative functions of wetland networks of multiple wetlands and flowpaths at the watershed scale. This model allows for combination of series of individual wetlands and flowpaths, as well as variation of topology (i.e. the spatial arrangement of wetlands and flowpaths), within the landscape.

The third study in this work investigated the effects of altering hydrologic connectivity in archetypal wetland landscapes on the natural flow regime of downstream waters. The model created in the second study was applied to scenarios in which the topology of archetypal wetland networks was varied. Results showed that simulating changes to hydrologic connectivity in the modeled archetypal headwater wetland complexes via altering wetlandscape topology affected characteristics of flows generated and propagated downstream. These results indicate that altering the hydrologic connectivity of headwater wetlandscapes affects the natural flow regimes produced therein, thereby influencing the integrity of downstream waters.

The findings of these works indicate that headwater wetlands are connected to downstream waters, they influence the flows generated and propagated to downstream waters, and therefore they influence the integrity of downstream waters. These findings have implications for vernal pools and other classes of headwater wetlands, and for headwater wetlandscapes in general.

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