Tim Covino

Redistribution of pyrogenic carbon from hillslopes to stream corridors following a large montane wildfire

Pyrogenic carbon (PyC) constitutes a significant fraction of organic carbon in most soils. However PyC soil stocks are generally smaller than what is expected from estimates of PyC produced from fire and decomposition losses, implying that other processes cause PyC loss from soils. Surface erosion has been previously suggested as one such process. To address this, following a large wildfire in the Rocky Mountains (CO, USA), we tracked PyC from the litter layer and soil, through eroded, suspended, and dissolved solids to alluvial deposits along river sides. We separated deposited sediment into high- and low-density fractions to identify preferential forms of PyC transport, and quantified PyC in all samples and density fractions using benzene polycarboxylic acid markers. A few months after the fire, PyC had yet to move vertically into the mineral soil and remained in the organic layer or had been transported off site by rainfall driven overland flow. During major storm events PyC was associated with suspended sediments in river water, and later identified in low-density riverbank deposits. Flows from an unusually long-duration and high magnitude rain storm either removed or buried the riverbank sediments approximately one year after their deposition. We conclude that PyC redistributes after wildfire in patterns that are consistent with erosion and deposition of low-density sediments. A more complete understanding of PyC dynamics requires attention to the interaction of post-fire precipitation patterns and geomorphological features that control surface erosion and deposition throughout the watershed. Index Terms: Carbon Cycling, Soils, Biogeochemistry.

Lateral inflows, stream‐groundwater exchange, and network geometry influence stream water composition

The role of stream networks and their hydrologic interaction with hillslopes and shallow groundwater in modifying and transporting watershed signals is an area of active research. One of the primary ways that stream networks can modify watershed signals is through spatially variable stream gains and losses, described herein as hydrologic turnover. We measured hydrologic gain and loss at the reach scale using tracer experiments throughout the Bull Trout watershed in the Sawtooth Mountains of Idaho. We extended the results of reach scale experiments to the stream network using empirical relationships between (1) watershed area and stream discharge and (2) stream discharge and percent stream water loss to the groundwater system. We thus incorporate linkages between (1) hillslopes and stream networks via lateral inflows and (2) stream networks and shallow groundwater via hydrologic exchange. We implemented these relationships within a concise analytical framework to simulate hydrologic turnover across stream networks and estimate the variable influence exerted by upstream reaches and streamflow source locations on stream water composition across stream networks. Application to six natural Sawtooth watersheds and seven synthetic watersheds with varying topographic structure and stream network geometry indicated that contributions to discharge from any upstream source depend on the magnitude of the initial input, but also on the distribution of hydrologic turnover occurring along the stream network. The evolution of stream water source compositions along stream networks was unique in each watershed due to the combination of watershed structure and stream network geometry. Our results suggest that a distributed representation of hydrologic turnover at the stream network scale can improve understanding of how the stream network can modify source water compositions along the stream.

Beaver‐mediated lateral hydrologic connectivity, fluvial carbon and nutrient flux, and aquatic ecosystem metabolism

River networks that drain mountain landscapes alternate between narrow and wide valley segments. Within the wide segments, beaver activity can facilitate the development and maintenance of complex, multi-thread planform. Because the narrow segments have limited ability to retain water, carbon, and nutrients, the wide, multi-thread segments are likely important locations of retention. We evaluated hydrologic dynamics, nutrient flux, and aquatic ecosystem metabolism along two adjacent segments of a river network in the Rocky Mountains, Colorado: 1) a wide, multi-thread segment with beaver activity; and, 2) an adjacent (directly upstream) narrow, single-thread segment without beaver activity. We used a mass balance approach to determine the water, carbon, and nutrient source-sink behavior of each river segment across a range of flows. While the single-thread segment was consistently a source of water, carbon, and nitrogen, the beaver impacted multi-thread segment exhibited variable source-sink dynamics as a function of flow. Specifically, the multi-thread segment was a sink for water, carbon, and nutrients during high flows, and subsequently became a source as flows decreased. Shifts in river-floodplain hydrologic connectivity across flows related to higher and more variable aquatic ecosystem metabolism rates along the multi-thread relative to the single-thread segment. Our data suggest that beaver activity in wide valleys can create a physically complex hydrologic environment that can enhance hydrologic and biogeochemical buffering, and promote high rates of aquatic ecosystem metabolism. Given the widespread removal of beaver, determining the cumulative effects of these changes is a critical next step in restoring function in altered river networks.

The influence of an in‐network lake on the timing, form, and magnitude of downstream dissolved organic carbon and nutrient flux

Within fluvial networks, lakes can be sinks or sources of dissolved organic carbon (DOC) and nutrients, yet the controls over sink-source behavior remain unclear. We investigated the influence that an in-network lake exerted on DOC and nutrient export. Our investigation consisted of: 1) injecting a conservative tracer to determine lake travel times and flow paths; 2) sampling lake inflow, outflow, and surrounding groundwater to determine water and nutrient budgets; and, 3) sampling internal lake profiles to ascertain in-lake physico-chemical patterns through time. Conservative tracer data indicated considerable in-lake retention and combined with inflow-outflow discharge measurements revealed a decoupling of kinematic and solute pulses. Nitrate (NO3) was the dominant form of dissolved inorganic nitrogen (DIN) at lake inflow whereas ammonium (NH4) became the dominant component at lake outflow. The lake was a sink for NO3-N and PO4, but a source for NH4-N, DON, TDN, and DOC. We observed hydrologic controls on DOC concentrations and export patterns, but redox controls on DIN dynamics. Our results indicate that lakes within fluvial networks can be sources of dissolved organic material and reduced nitrogen (NH4) while simultaneously being sinks for NO3-N and PO4-P. Determining controls on sink-source behavior and the cumulative effect of lakes on DOC and nutrient budgets is a necessary first step toward improved understanding of the role of lakes in network- to regional-scale dynamics. This article is protected by copyright. All rights reserved.

Evaluating Controls on Nutrient Retention and Export in Wide and Narrow Valley Segments of a Mountain River Corridor

Over the past few decades, nitrate-nitrogen (NO3-N) concentrations have increased within streams of the central Rockies, a pattern linked to regional N deposition trends. As NO3-N concentrations increase, in-stream biological demand may become saturated and stream N export may increase. In mountain landscapes, streams generally flow through steep, narrow valleys with limited riparian area and strong stream-hillslope connectivity. Interspersed between the narrow valleys are wide segments where substantial floodplain riparian areas can develop. Here, we coupled measures of stream reach NO3-N flux balances with nutrient enrichment experiments along two stream reaches of contrasting valley morphology in Rocky Mountain National Park. The stream reaches were (1) a narrow valley segment with limited floodplain riparian area and (2) a longitudinally adjacent (directly downstream) wide valley segment with extensive floodplain riparian area. We found that in-stream biological uptake of added NO3-N was limited in both segments, presumably as a consequence of saturating conditions. Assessment of mass flux indicated that the narrow valley segment was a consistent source of water and NO3-N across flow states, while the wide segment was a sink at high flow and a source at low flow. Due to low in-stream biological retention, gross gains and losses of water and NO3-N to and from the stream exerted primary constraint on segment mass balances. Our results suggest that the exchange of water and nutrients between the stream and adjacent landscape can exert strong control on reach-scale nutrient export, particularly in streams experiencing or approaching N saturation.

Nutrient uptake in a simplified stream channel: Experimental manipulation of hydraulic residence time and transient storage: Nutrient uptake in a tropical stream channel

Departamento de Hidráulica e Saneamento, Escola de Engenharia de São Carlos, Universidade de São Paulo, São Carlos, Brazil Department of Ecosystem Science and Sustainability, Colorado State University, Fort Collins, CO, USA Global Water Center and Department of Biology, University of Nevada, Reno, NV, USA Division of Biology, Kansas State University, Manhattan, KS, USA Correspondence Davi Gasparini Fernandes Cunha, Departamento de Hidráulica e Saneamento, Escola de Engenharia de São Carlos, Universidade de São Paulo, São Carlos, Brazil. Email: davig@sc.usp.br

Form-based river restoration decreases wetland hyporheic exchange: lessons learned from the Upper Colorado River: Form-based river restoration decreases wetland hyporheic exchange

Restoration of river–wetland systems to recover lost ecosystem services and restore consistent flood regimes is commonly directed at modifying in-channel storage and hyporheic exchange. Here, we monitored the hydrologic response to channel realignment in a montane river–wetland system by comparing preand post-restoration measurements. In 2015, an earthen berm and 190m segment of the Upper Colorado River were constructed to consolidate flow from multiple channels into the historic thalweg. We injected a sodium chloride tracer during baseflow and used mass-balance calculations and electrical resistivity imaging to assess changes in near-channel hyporheic exchange. Results indicate a decrease in hyporheic exchange within the wetland due to lost complexity along the consolidated flow path. Subsurface complexity appears to control hyporheic exchange more than surface complexity. Flow consolidation increased the area-adjusted wetland water yield by 231mm, indicating a loss of wetland water storage capacity. One year of post-restoration monitoring suggests that the form-based channel restoration directed at consolidating flow into a single thread adversely affected the hyporheic exchange functioning in the pre-restoration system. Results from this case study are applicable to restoration planners as they consider the effects of form-based projects on water storage capacity in similar systems.