Logjam Characteristics as Drivers of Transient Storage in Headwater Streams
A. Marshall1, X. Zhang2, A.H. Sawyer2, E. Wohl1, and K. Singha3
1Department of Geosciences, Colorado State University, Fort Collins, CO, USA
2School of Earth Sciences, The Ohio State University, Columbus, OH, USA
3Colorado School of Mines, Hydrologic Science and Engineering Program, Golden, CO, USA
Corresponding author: Anna Marshall (amarsh01@colostate.edu)
Key Points:
Abstract
Logjams in a stream create backwater conditions and locally force water to flow through the streambed, creating zones of transient storage within the surface and subsurface of a stream. We investigate the relative importance of logjam distribution density, logjam permeability, and discharge on transient storage in a simplified experimental channel. We use physical flume experiments in which we inject a salt tracer, monitor fluid conductivity breakthrough curves in surface water, and use breakthrough-curve skew to characterize transient storage. We then develop numerical models in HydroGeoSphere to reveal flow paths through the subsurface (or hyporheic zone) that contribute to some of the longest transient-storage timescales. In both the flume and numerical model, we observe an increase in backwater and hyporheic exchange at logjams. Observed complexities in transient storage behavior may depend largely on surface water flow in the backwater zone. As expected, multiple successive logjams provide more pervasive hyporheic exchange by distributing the head drop at each jam, leading to distributed but shallow flow paths. Decreasing the permeability of a logjam or increasing the discharge both facilitate more surface water storage and elevate the surface water level upstream of a logjam, thus increasing hyporheic exchange. Multiple logjams with low permeability result in the greatest magnitude of transient storage, suggesting that this configuration maximizes solute retention in backwater zones, while hyporheic exchange rates also increase. Understanding how logjam characteristics affect solute transport through both the channel and hyporheic zone has important management implications for rivers in forested, or historically forested, environments.
1 Introduction
Spatial heterogeneity in flow paths within a river corridor drives stream solute exchange between mobile areas of the channel and relatively immobile transient storage zones. Transient storage can be generally segregated into surface transient storage—where water flows slowly through recirculation zones and stagnant areas of low velocity—and subsurface transient storage (controlled in part by hyporheic exchange—where stream water flows through the subsurface and returns to the channel). Transient storage has numerous benefits to river corridor ecosystem services and processes including i) increased biogeochemical cycling (Fischer et al., 2005; Battin et al., 2008; Tonina & Buffington, 2009; Harvey & Gooseff, 2015; Marttila et al., 2018); ii) nutrient and pollutant processing (Harvey & Wagnert, 2000; Hall et al., 2002; Ensign & Doyle, 2005; Stewart et al., 2011); iii) increased habitat diversity and thermal refugia (Mulholland et al., 2004; Hester & Gooseff, 2010); and iv) flow attenuation (Herzog et al., 2018). Transient storage can be increased by morphologic and geologic features that create spatial heterogeneity in water velocity and drive alternate patterns of downwelling and upwelling along the bed. Examples of such features include bedforms and other variations in channel cross-sectional geometry (Bencala, 1983; Harvey & Bencala, 1993; Kasahara & Wondzell, 2003; Ensign & Doyle, 2005; Gooseff et al., 2007), logjams (Hester & Doyle, 2008; Sawyer et al., 2011; Marttila et al., 2018; Ader et al., 2021), and variations in alluvial thickness and grain-size distribution (Harvey et al., 1996). Here, we focus on the effects of logjams as an important morphologic element that creates both surface transient storage in the channel (for example, backwater zones) and subsurface transient storage in porous media (for example, hyporheic exchange).
Logjams directly enhance transient storage in a number of ways. Logjams obstruct flow and increase hydraulic resistance within the channel, thus creating hydraulic head gradients that drive hyporheic exchange. Logjams directly influence surface transient storage by creating low-velocity zones within the channel (Gippel, 1995); enhancing the formation of backwater pools (Richmond & Fausch, 1995; Kaufmann & Faustini, 2012; Beckman & Wohl, 2014; Livers & Wohl, 2016); creating scour pools that enhance residual pool volume (Fausch & Northcote, 1992; Ensign & Doyle, 2005; Mao et al., 2008); and creating marginal eddies (Zhang et al., 2019).
Logjams also indirectly affect surface and subsurface transient storage by increasing the erosion and deposition of sediment (Wohl & Scott, 2017). Logjams locally enhance entrainment of bed material and erosion of the channel bed and banks (Buffington et al., 2002). Studies of the effects of logjams on floodplain-sediment dynamics emphasize how the obstructions created by logjams can result in changes in bedforms via overbank flows and vertical accretion or bank erosion, channel avulsion, and formation of secondary channels (e.g., Sear et al., 2010; Wohl & Scott, 2017). Logjams commonly create high spatial variability in average bed grain size and alluvial thickness upstream and downstream of a jam (Massong & Montgomery, 2000). Advective pumping, induced by streamflow over a spatially heterogeneous and permeable bed, leads to a distribution of pore-water flow paths in the streambed (Wörman et al., 2002), which in turn enhances the magnitude of subsurface transient storage via hyporheic exchange (Lautz et al., 2006; Hester & Doyle, 2008; Fanelli & Lautz, 2008; Sawyer et al., 2011; Sawyer & Cardenas, 2012).
As might be expected, previous work indicates that greater roughness (e.g., Harvey et al., 2003) and spatial heterogeneity within a channel (e.g., Gooseff et al., 2007) equate to greater potential for transient storage. A growing body of research describes wood as a driver of channel spatial heterogeneity (e.g., Buffington & Montgomery, 1999; Collins et al., 2012; Faustini & Jones, 2003) and as a driver of transient storage (e.g., Mutz et al., 2007; Sawyer et al., 2011; Sawyer & Cardenas, 2012; Kaufmann & Faustini, 2012; Doughty et al. 2020; Ader et al., 2021; Wilhelmsen et al., 2021). Recent work used bulk electrical conductivity (Doughty et al., 2020) and fluid electrical conductivity (Ader et al., 2021) to examine surface and subsurface transient storage in a small stream with and without the presence of logjams and found that the direct presence of wood increases transient storage and does so at a greater magnitude than other geomorphic variables, such as bedform dimensions. Wilhelmsen et al. (2021) combined flume experiments with a numerical model to analyze the effects of jam complexity, in combination with channel planform complexity, on the hyporheic flow regime of small streams. Their numerical simulations suggest that logjams decrease the turnover length that stream water travels before interacting with the hyporheic zone by an order of magnitude and that the broadest range of hyporheic residence times arise where logjams and multiple channel threads co-occur. While these field and modeling studies have shaped our understanding of transient storage around logjams and channel morphologies of varying complexities, an opportunity exists to test the effects of logjam characteristics on transient storage patterns. No study, to our knowledge, has used empirical data to comparatively examine transient storage in a stream as the number of logjams increases (e.g., distribution density) nor have any addressed how the structure of jams (e.g., permeability) influences transient storage.
These characteristics of logjams are important to understand because in natural settings, jams vary in size, shape, and permeability depending on the abundance and composition of large wood and coarse particulate organic matter (see terminology in Table 1). Quantifying logjam characteristics in the field has proved challenging (Manners et al. 2007, Livers et al., 2020) and physical and numerical modeling approaches are commonly used to further constrain field variables. Recent work explores the influence of jam sorting and organizational structure on logjam permeability (Spreitzer et al., 2019) and resulting hydraulic impacts (Schalko et al., 2018; Ismail et al, 2020; Follett et al., 2021). A small number of physical modeling studies have relied on natural wood to study hydraulics and geomorphology (e.g., Beebe 2000; Mutz et al., 2007; Schalko 2020; Schalko & Weitbrecht, 2021; Spreitzer et al., 2021), but none have used natural wood to examine logjam accumulation characteristics (Friedrich et al., 2022). Knowledge of how logjam characteristics influence hydrologic function is pertinent to river management as wood is increasingly used to restore a more natural hydrologic function to rivers (Roni et al., 2014; Grabowski et al., 2019). Limited understanding of how logjam characteristics relate to specific hydrologic effects constrains our ability to maximize functions of constructed logjams to promote ecosystem services provided by transient storage.
Here, we address some of the gaps in understanding the relationship between logjam characteristics and transient storage at varying discharges and logjam configurations. Our study objective is to assess the relative response of transient storage to binary changes in logjam permeability (high versus low), logjam distribution density (single versus multiple jams along a given length of channel), and discharge (high versus low). We achieve this objective through a two-part approach. We use flume experiments, specifically focusing on measuring salt breakthrough curves in the channel, to address faster timescales of transient storage. Because breakthrough curves measured in surface water often fail to capture some of the longer residence times in the hyporheic zone (Harvey et al., 1996) and do not reveal spatial information about flow paths, we also numerically simulated coupled surface-subsurface flow in flume experiments to resolve longer flow paths through the subsurface. We treated the jams themselves as an extension of the porous medium with adjustable permeability. We test four hypotheses: H1) increasing logjam longitudinal distribution density enhances transient storage; H2) increasing the permeability of a single logjam enhances transient storage; H3) a single low-permeability logjam creates a comparable increase in transient storage to multiple high-permeability logjams; and H4) transient storage increases at higher discharge for all scenarios.
Table 1. Wood terminology definitions.