The role of valley confinement on water, carbon, and nitrogen fluxes in North Saint Vrain Creek, Colorado

From May - October 2015, I quantified water, carbon, and nitrogen fluxes along two contrasting geomorphic segments of North Saint Vrain Creek in Rocky Mountain National Park: a single-thread confined reach ("confined segment") and a low-gradient, multi-thread beaver meadow wetland ("unconfined segment") located directly downstream. The data showed that while the confined segment was a consistent source of water, carbon, and nitrogen, the unconfined segment exhibited variable source-sink dynamics as a function of runoff. Notably, the majority of retention along the unconfined segment occurred during high flow periods, when export rates along the confined segment were highest.

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Why care?

While unconfined valley bottoms in the headwaters of Rocky Mountain National Park constitute a small portion of the total river length, the contribution of these features to network retention can be substantial. Nevertheless, land management practices such as channelization and the construction of levees often alter unconfined valley bottoms and can create environments analogous to more confined river segments. Given the growing potential for drought and flooding and elevated nutrient loading to fluvial systems, hydrologic and biogeochemical buffering is increasingly important in order to maintain high water tables during droughts, attenuate flood pulses, and store, transform, and remove carbon and nitrogen loads from the river network.

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The influence of lateral (river-floodplain) hydrologic connectivity on aquatic ecosystem metabolism

In an unconfined beaver meadow wetland segment of North Saint Vrain Creek, Rocky Mountain National Park, there are a number of side-channels and ponds along the floodplain with varying degrees of hydrologic connectivity with the main channel (i.e., always connected, sometimes connected, and never connected) from high to low flows. In these side-channels and ponds, I used diurnal changes in dissolved oxygen to quantify ecosystem metabolism metrics gross primary productivity (GPP) and ecosystem respiration (ER) across flows. From May - October, mean GPP and ER rates were highest in a side-channel with intermittent surface water connections with the main channel. Although low flow/low connectivity periods can enhance GPP and ER due to extended contact between water, sediments, and microbes, episodic overbank flooding is likely required to replenish resource supplies and maintain high levels of ecosystem processing.

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Why care?

Lateral (river-floodplain) hydrologic connectivity along unconfined river segments can connect nutrient sources and sinks to enhance aquatic ecosystem metabolism and associated stream health. However, when unconfined segments are altered by activities such as beaver trapping, channelization, dredging, or artificial levee construction, previously intermittently connected side-channels can become permanently disconnected from the main channel. These river-floodplain alterations can decrease metabolism and associated nutrient retention and processing.

 

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Email: pamela.wegener@colostate.edu

Phone: (516) 302-5603

Research Group: Watershed Hydrology and Biogeochemistry at CSU