Natural dams and biogeochemistry at the river network scale: implications for water quality

Start Date: 2014-03-01 End Date: 2016-02-29
Principal Investigators: Denise Burchsted, Mark Green, Jennifer Jacobs, Wil Wollheim

Abstract: Water quality in New England rivers is dramatically affected by dams created both by humans and by other natural agents such as beavers or log jams. These dams break a critical assumption embedded in much of our infrastructure and watershed modeling, which is that rivers are continuous systems. In river systems without direct modern human management, natural dams create frequent discontinuities; they occur at frequencies up to ten per kilometer and their impoundments can extend laterally from one valley wall to the other. The impoundments created by these dams have low oxygen levels that dramatically alter biogeochemical processing such as slowing or eliminating nitrification and increasing denitrification. Given the new abundance of these features in river networks, due to the recent regrowth of riparian forests that generate large logs and the recent recolonization of river networks by beavers, it is increasingly important to understand the role of natural dams in biogeochemical state and, by extension, in the water quality of New England’s rivers. The proposed research will address this need by considering the overall research question what is the difference in biogeochemical regime between free-flowing river reaches and river reaches associated with natural dams, and what is the extent of this difference at the river network scale? To address this question, we propose to both systematically collect site-scale biogeochemistry parameters along river networks that include free-flowing reaches and natural dams and to examine the landscape-scale parameters that control the presence of natural dams. The three specific research questions follow: (1) Can free-flowing river reaches and river reaches associated with natural dams be classified according to biogeochemical regime? (2) What is the nature of the transition in biogeochemical regime downstream of a natural dam? (3) Which landscape and demographic factors control their presence and frequency of natural dams? This proposal requests funding for the field research that addresses questions 1 and 2; this funding request complements the GIS investigation, addressing question 3, that would be covered partially by the request but primarily by non-federal funds. The field research will collect both high spatial resolution and high temporal resolution data. High spatial resolution data will be collected during stream walk surveys that will measure temperature, pH, dissolved oxygen, conductivity, oxidation reduction potential, stable water isotopes, and channel shape. These data will be collected at regular intervals along free-flowing channels and, where the survey encounters dams, at several points associated with the dams. High temporal resolution data will be collected by ten data logger arrays—with a temperature, stage, and conductivity sensor in each array—and by 27 additional temperature loggers; these will all be used to compare variability within and across beaver meadows, ponds and free-flowing channels. Four of the arrays will be provided by the EPSCoR LoVoTECS project and all of the data from the arrays will be incorporated into that data set. The complementary GIS research will classify reaches in a river network according to their modification by dams (e.g., free-flowing vs. impounded) and generate a predictive model of reach class based on landscape characteristics. The requested funds will primarily support a Master’s student to lead field data collection. This will not only generate much-needed research but will also strengthen a nascent bond for joint student research between Keene State College and Antioch University of New England, increasing the research training of undergraduate and graduate students in New Hampshire.