An official website of the United States government
Abstract Most terrestrial allochthonous organic matter enters river networks through headwater streams during high flow events. In headwaters, allochthonous inputs are substantial and variable, but become less important in streams and rivers with larger watersheds. As allochthonous dissolved organic matter (DOM) moves downstream, the proportion of less aromatic organic matter with autochthonous characteristics increases. How environmental factors converge to control this transformation of DOM at a continental scale is less certain. We hypothesized that the amount of time water has spent travelling through surface waters of inland systems (streams, rivers, lakes, and reservoirs) is correlated to DOM composition. To test this hypothesis, we used established river network scaling relationships to predict relative river network flow‐weighted travel time (FWTT) of water for 60 stream and river sites across the contiguous United States (3090 discrete samples over 10 water years). We estimated lentic contribution to travel times with upstream in‐network lake and reservoir volume. DOM composition was quantified using ultraviolet and visible absorption and fluorescence spectroscopy. A combination of FWTT and lake and reservoir volume was the best overall predictor of DOM composition among models that also incorporated discharge, specific discharge, watershed area, and upstream channel length. DOM spectral slope ratio (R2 = 0.77) and Freshness Index (R2 = 0.78) increased and specific ultraviolet absorbance at 254 nm (R2 = 0.68) and Humification Index (R2 = 0.44) decreased across sites as a function of FWTT and upstream lake volume. This indicates autochthonous‐like DOM becomes continually more dominant in waters with greater FWTT. We assert that river FWTT can be used as a metric of the continuum of DOM composition from headwaters to rivers. The nature of the changes to DOM composition detected suggest this continuum is driven by a combination of photo‐oxidation, biological processes, hydrologically varying terrestrial subsidies, and aged groundwater inputs.
more »
« less
The Abundance, Size, and Spacing of Lakes and Reservoirs Connected to River Networks
Abstract Descriptions of river network topology do not include lakes/reservoirs that are connected to rivers. We describe the properties and scaling patterns of river network topology across the contiguous United States: how lake/reservoir abundance, median lake/reservoir size, and median lake/reservoir spacing change with river size. Typically, lake/reservoir abundance decreases, median lake/reservoir size increases, but median lake/reservoir spacing is uniform across river size. There is a characteristic lake/reservoir size of 0.01–0.05 km2and a characteristic lake/reservoir spacing of 1–5 km that shifts to 27–61 km in larger rivers. Climate explains more of the variance in river network topology than both glacial history and constructed reservoirs. Our results provide conceptual models for building river network topologies to assess how lake/reservoir abundance, size, and spacing effect the transport, storage, and cycling of water, materials, and organisms across networks.
more »
« less
- Award ID(s):
- 1806983
- PAR ID:
- 10460392
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 46
- Issue:
- 5
- ISSN:
- 0094-8276
- Page Range / eLocation ID:
- p. 2592-2601
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Mountain rivers often receive sediment in the form of episodic, discrete pulses from a variety of natural and anthropogenic processes. Once emplaced in the river, the movement of this sediment depends on flow, grain size distribution, and channel and network geometry. Here, we simulate downstream bed elevation changes that result from discrete inputs of sediment (∼10,000 m3), differing in volume and grain size distribution, under medium and high flow conditions. We specifically focus on comparing bed responses between mixed and uniform grain size sediment pulses. This work builds on a Lagrangian, bed‐material sediment transport model and applies it to a 27 km reach of the mainstem Nisqually River, Washington, USA. We compare observed bed elevation change and accumulation rates in a downstream lake to simulation results. Then we investigate the magnitude, timing, and persistence of downstream changes due to the introduction of synthetic sediment pulses by comparing the results against a baseline condition (without pulse). Our findings suggest that bed response is primarily influenced by the sediment‐pulse grain size and distribution. Intermediate mixed‐size pulses (∼50% of the median bed gravel size) are likely to have the largest downstream impact because finer sizes translate quickly and coarser sizes (median bed gravel size and larger) disperse slowly. Furthermore, a mixed‐size pulse, with a smaller median grain size than the bed, increases bed mobility more than a uniform‐size pulse. This work has important implications for river management, as it allows us to better understand fluvial geomorphic responses to variations in sediment supply.more » « less
-
Abstract Around the world, long‐term changes in the timing and magnitude of streamflow are testing the ability of large managed water resource systems constructed in the 20th century to continue to meet objectives in the 21st century. Streamflow records for unregulated rivers upstream of reservoirs can be combined with records downstream of reservoirs using a paired‐watershed framework and concepts of water resource system performance to assess how reservoir management has responded to long‐term change. Using publicly available data, this study quantified how the intra‐annual timing of inflows and outflows of 25 major reservoirs has shifted, how management has responded, and how this has influenced reliability and vulnerability of the water resource system in the 668,000 km2Columbia River basin from 1950 to 2012. Reservoir inflows increased slightly in early spring and declined in late spring to early fall, but reservoir outflows increased in late summer from 1950 to 2012. Average inflows to reservoirs in the low flow period exceeded outflows in the1950s, but inflows are now less than outflows. Reservoirs have increased hedging, that is, they have stored more water during the spring, in order to meet the widening gap between inflows and outflows during the summer low flow period. For a given level of reliability (the fraction of time flow targets were met), vulnerability (the maximum departure from the flow target) was greater during periods with lower than average inflows. Thus, the water management system in this large river basin has adjusted to multi‐decade trends of declining inflows, but vulnerability, that is, the potential for excess releases in spring and shortfalls in summer, has increased. This study demonstrates the value of combining publicly available historical data on streamflow with concepts from paired‐watershed analyses and metrics of water resource performance to detect, evaluate, and manage water resource systems in large river basins.more » « less
-
Abstract Riverine fluxes of carbon and inorganic nutrients are increasing in virtually all large permafrost-affected rivers, indicating major shifts in Arctic landscapes. However, it is currently difficult to identify what is causing these changes in nutrient processing and flux because most long-term records of Arctic river chemistry are from small, headwater catchments draining <200 km2or from large rivers draining >100,000 km2. The interactions of nutrient sources and sinks across these scales are what ultimately control solute flux to the Arctic Ocean. In this context, we performed spatially-distributed sampling of 120 subcatchments nested within three Arctic watersheds spanning alpine, tundra, and glacial-lake landscapes in Alaska. We found that the dominant spatial scales controlling organic carbon and major nutrient concentrations was 3–30 km2, indicating a continuum of diffuse and discrete sourcing and processing dynamics. These patterns were consistent seasonally, suggesting that relatively fine-scale landscape patches drive solute generation in this region of the Arctic. These network-scale empirical frameworks could guide and benchmark future Earth system models seeking to represent lateral and longitudinal solute transport in rapidly changing Arctic landscapes.more » « less
-
Abstract River avulsions generate catastrophic floods that threaten communities, ecosystems, and infrastructure worldwide. Alluvial ridges—elevated regions of near‐channel topography—are thought to precede avulsions, yet their spatial patterns and relationship to avulsion impact remain poorly understood. We analyzed pre‐event topographic cross‐sections from 14 rivers to quantify avulsion potential , a metric combining ridge height and slope relative to the channel. Our analysis reveals that varies downstream and defines distinct alluvial ridge segments. We identify two characteristic length scales: a longer‐wavelength complex ( 30 km) composed of shorter ridge segments ( 8 km). Segments with 2 correspond to 73% of observed avulsion activity locations ( = 37). Avulsion activity length scales linearly with ; evidence that ridge geometry controls avulsion activity size. These characteristic scales define both the minimum downstream extent of potential impact zones and the spacing between avulsion‐prone reaches , enabling improved hazard assessment.more » « less
