Jon Schwenk

The change of nature and the nature of change in agricultural landscapes: Hydrologic regime shifts modulate ecological transitions

Hydrology in many agricultural landscapes around the world is changing in unprecedented ways due to the development of extensive surface and subsurface drainage systems that optimize productivity. This plumbing of the landscape alters water pathways, timings, and storage, creating new regimes of hydrologic response and driving a chain of environmental changes in sediment dynamics, nutrient cycling, and river ecology. In this work we non-parametrically quantify the nature of hydrologic change in the Minnesota River Basin, an intensively managed agricultural landscape, and study how this change might modulate ecological transitions. During the growing season when climate effects are shown to be minimal, daily streamflow hydrographs exhibit sharper rising limbs and stronger dependence on the previous-day precipitation. We also find a changed storage-discharge relationship and show that the artificial landscape connectivity has most drastically affected the rainfall-runoff relationship at intermediate quantiles. Considering the whole year, we show that the combined climate and land-use change effects reduce the inherent nonlinearity in the dynamics of daily streamflow, perhaps reflecting a more linearized engineered hydrologic system. Using a simplified dynamic interaction model that couples hydrology to river ecology, we demonstrate how the observed hydrologic change and/or the discharge-driven sediment generation dynamics may have modulated a regime shift in river ecology, namely extirpation of native mussel populations. We posit that such non-parametric analyses and reduced complexity modeling can provide more insight than highly parameterized models and can guide development of vulnerability assessments and integrated watershed management frameworks. This article is protected by copyright. All rights reserved.

The life of a meander bend: Connecting shape and dynamics via analysis of a numerical model

Analysis of bend-scale meandering river dynamics is a problem of theoretical and practical interest. This work introduces a method for extracting and analyzing the history of individual meander bends from inception until cutoff (called “atoms”) by tracking backward through time the set of two cutoff nodes in numerical meander migration models. Application of this method to a simplified yet physically based model provides access to previously unavailable bend-scale meander dynamics over long times and at high temporal resolutions. We find that before cutoffs, the intrinsic model dynamics invariably simulate a prototypical cutoff atom shape we dub simple. Once perturbations from cutoffs occur, two other archetypal cutoff planform shapes emerge called long and round that are distinguished by a stretching along their long and perpendicular axes, respectively. Three measures of meander migration—growth rate, average migration rate, and centroid migration rate—are introduced to capture the dynamic lives of individual bends and reveal that similar cutoff atom geometries share similar dynamic histories. Specifically, through the lens of the three shape types, simples are seen to have the highest growth and average migration rates, followed by rounds, and finally longs. Using the maximum average migration rate as a metric describing an atoms dynamic past, we show a strong connection between it and two metrics of cutoff geometry. This result suggests both that early formative dynamics may be inferred from static cutoff planforms and that there exists a critical period early in a meander bends life when its dynamic trajectory is most sensitive to cutoff perturbations. An example of how these results could be applied to Mississippi River oxbow lakes with unknown historic dynamics is shown. The results characterize the underlying model and provide a framework for comparisons against more complex models and observed dynamics.

Coupling freshwater mussel ecology and river dynamics using a simplified dynamic interaction model

Freshwater faunal diversity and abundance have declined dramatically worldwide, concurrent with changes in streamflow and sediment loads in rivers. Cumulative effects and interdependencies of chronic covarying environmental stressors can obscure causal linkages that may be controlling the population dynamics of longer-lived freshwater fauna, such as mussels. To understand changes in long-term mussel population density, we developed a dynamic, process-based interaction model that couples streamflow, suspended sediment, phytoplankton, and mussel abundance under the hypothesis that chronic exposure to increased suspended sediment and food limitation are the primary factors controlling native mussel population density in a midwestern USA agricultural river basin. We calibrated and validated the model with extensive survey data from multiple time periods and used it to evaluate changes in mussel abundance at a subbasin scale over decades. We evaluated sensitivity of simulated mussel densities across a range of mortality rates and initial population densities. In scenarios representing altered sediment concentrations, such as might occur with climate or landuse-induced changes in streamflow or sediment generation rates, mussel population density showed critical threshold responses to long-term changes in suspended sediment concentration. This model of mussel population density can be used to test hypotheses about limiting factors, identify priority locations for restoration activities, and evaluate the effects of climate- or landuse-change scenarios.

High spatiotemporal resolution of river planform dynamics from Landsat: The RivMAP toolbox and results from the Ucayali River

Quantifying planform changes of large and actively migrating rivers such as those in the tropical Amazon at multidecadal time scales, over large spatial domains, and with high spatiotemporal frequency is essential for advancing river morphodynamic theory, identifying controls on migration, and understanding the roles of climate and human influences on planform adjustments. This paper addresses the challenges of quantifying river planform changes from annual channel masks derived from Landsat imagery and introduces a set of efficient methods to map and measure changes in channel widths, the locations and rates of migration, accretion and erosion, and the space-time characteristics of cutoff dynamics. The techniques are assembled in a comprehensive MATLAB toolbox called RivMAP (River Morphodynamics from Analysis of Planforms), which is applied to over 1500 km of the actively migrating and predominately meandering Ucayali River in Peru from 1985 to 2015. We find multiscale spatial and temporal variability around multidecadal trends in migration rates, erosion and accretion, and channel widths revealing a river dynamically adjusting to sediment and water fluxes. Confounding factors controlling planform morphodynamics including local inputs of sediment, cutoffs, and climate are parsed through the high temporal analysis.

Comment on “Climate and agricultural land use change impacts on streamflow in the upper midwestern United States” by Satish C. Gupta et al.

The paper “Climate and agricultural land use change impacts on streamflow in the upper midwestern United States” by Satish C. Gupta, Andrew C. Kessler, Melinda K. Brown, and Francis Zvomuya (hereafter referred to as Gupta et al.) purports to evaluate “the relative importance of changes in precipitation and LULC (land use, land cover) on streamflow in 29 Hydrologic Unit Code 008 watersheds in the Upper Midwestern United States.” However, as we report here, the approach used by Gupta et al. is wholly inadequate for making such an evaluation. Gupta et al. use strong language to criticize other studies and imply a level of certainty that goes well beyond, and in some cases is entirely unsupported by, the results they have presented. We take this opportunity to point out several critical flaws in their study. This article is protected by copyright. All rights reserved.

Meander cutoffs nonlocally accelerate upstream and downstream migration and channel widening

The hydrologic and sediment dynamics within and near cutoffs have long been studied, establishing them as effective agents of rapid local geomorphic change. However, the morphodynamic impact of individual cutoffs at the reachwide scale remains unknown, mainly due to insufficient observations of channel adjustments over large areal extents and at high temporal frequency. Here we show via annually-resolved, Landsat-derived channel masks of the dynamic meandering Ucayali River in Peru that cutoffs act as perturbations that nonlocally accelerate river migration and drive channel widening both up- and downstream of the cutoff locations. By tracking planform changes of individual meander bends near cutoffs, we find that the downstream distance of cutoff influence scales linearly with the length of the removed reach. The discovery of nonlocal cutoff influence supports the hypothesis of “avalanche”-type behavior in meander cutoff dynamics and presents new challenges in modeling and prediction of rivers’ self-adjusting responses to perturbations.

Are process nonlinearities encoded in meandering river planform morphology

Meandering river planform evolution is driven by the interaction of local nonlinear processes and cutoff dynamics. Despite the known nonlinear dynamics governing the evolution of meandering rivers, previous attempts have found at most a weak signature of these process nonlinearities on the static meander planform morphologies (form nonlinearities). In this work, we present a framework to measure form nonlinearity from centerline curvature signals and unambiguously quantify its presence in both a numerically simulated meandering river and three natural rivers. The degree of nonlinearity (DNL) metric is introduced to measure the strength of form nonlinearities embedded in the centerlines. The DNLs evolution through time is computed for annual observations over 30 years of an active, tropical meandering river and for the simulated centerline to understand how cutoffs and bend growths affect form nonlinearity. We find that although cutoffs reduce the overall form nonlinearity, they also act as a source of nonlinearity themselves by creating scales that contribute disproportionately to DNL.