Noah J. Finnegan

Controls on the channel width of rivers: Implications for modeling fluvial incision of bedrock

On the basis of the Manning equation and basic mass conser- vation principles, we derive an expression for scaling the steady- state width (W) of river channels as a function of discharge (Q), channel slope (S), roughness (n), and width-to-depth ratio (a): W 5 (a( a1 2) 2/3 ) 3/8 Q 3/8 S 23/16 n 3/8 . We propose that channel width-to- depth ratio, in addition to roughness, is a function of the material in which the channel is developed, and that where a river is con- fined to a given material, width-to-depth ratio and roughness can be assumed constant. Given these simplifications, the expression emulates traditional width-discharge relationships for rivers incis- ing bedrock with uniformly concave fluvial long profiles. More sig- nificantly, this relationship describes river width trends in terrain with spatially nonuniform rock uplift rates, where conventional discharge-based width scaling laws are inadequate. We suggest that much of observed channel width variability in river channels confined by bedrock is a simple consequence of the tendency for water to flow faster in steeper reaches and therefore occupy smaller channel cross sections. We demonstrate that using conventional scaling relationships for channel width can result in underestima- tion of stream-power variability in channels incising bedrock and that our model improves estimates of spatial patterns of bedrock incision rates.

Coupling of rock uplift and river incision in the Namche Barwa-Gyala Peri massif, Tibet

Geodynamic modeling demonstrates the strong potential for erosion to influence the pattern and style of deformation in active mountain belts, but field studies yield conflicting views on the importance of erosion in influencing orogenesis. Here we compare patterns in river power, inferred excess fluvial-transport capacity, topographic relief, precipitation, and mineral-cooling ages to assess the coupling between surface erosion and rock uplift within the vicinity of the Namche Barwa–Gyala Peri massif, an active antiformal structure within the eastern Himalayan syntaxis. Our rich and dense data set reveals a tight spatial correspondence of fluvial incision potential, high relief, and young cooling ages. The spatial coincidence is most easily explained by a sustained balance between rock uplift and denudation driven by river incision over at least the last ∼1 m.y. The Yarlung Tsangpo–Brahmaputra River is the largest and most powerful river in the Himalaya, and two lines of evidence point to its active role in the dynamic interaction of local erosion, rock uplift, thermal weakening of the lithosphere, and deformation: (1) Whereas along the rest of the Himalayan front, high relief and high rock uplift rates are essentially continuous, the high relief and rapid exhumation in the syntaxis is restricted to a “bulls-eye” pattern exactly where the largest river in the Himalaya, the Yarlung Tsangpo–Brahmaputra, has the most energy per unit area available to erode its channel and transport sediment. (2) The location of rapid incision on the Yarlung Tsangpo–Brahmaputra has been pinned for at least 1 m.y., and without compensatory uplift of the Namche Barwa–Gyala Peri massif during this time the river would have eroded headward rapidly, incising deeply into Tibet.

Episodic bedrock strath terrace formation due to meander migration and cutoff

In order to explore mechanisms of bedrock terrace formation, we have developed a numerical model that couples vertical river incision and meandering. Model results illustrate that flights of unpaired strath terraces can form purely from the internal dynamics of bedrock river meandering in vertically incising channels. Specifically, knickpoints that propagate upstream following meander cutoffs enhance vertical incision, whereas channel lengthening and corresponding slope reduction during meander growth suppresses vertical incision. Analysis of topography from the Smith River, Oregon, USA, suggests terrace formation by this mechanism. Our results introduce an alternative mechanism to climatic or tectonic forcing, namely inherent instability triggered by meander growth and cutoff, that explains both oscillations in rates of vertical bedrock incision and the formation of longitudinally traceable, unpaired bedrock terraces. In addition, our results point to simple topographic criteria for identifying internally generated fluvial bedrock terraces.

A seismic signature of river bedload transport during storm events

GEOPHYSICAL RESEARCH LETTERS, VOL. 38, L13407, doi:10.1029/2011GL047759, 2011 A seismic signature of river bedload transport during storm events Leslie Hsu, 1,2 Noah J. Finnegan, 1 and Emily E. Brodsky 1 Received 12 April 2011; revised 3 June 2011; accepted 3 June 2011; published 14 July 2011. [ 1 ] Seasonal patterns in high frequency seismic waves near rivers can record energy transmitted to the river bed from particle impacts during bedload transport. Here we show that single storm events in a river can also be observed seismically. We analyzed the high frequency seismic noise in a reach of the Cho‐Shui (Zhuoshui ˇ ) River in central Taiwan and made detailed observations during individual storm events. Discharge, derived from a water level gage 4.25 km from the seismometer, is highly variable due to typhoons. We found a correlation between seismic amplitude and discharge that differs on the rising and falling limbs of three storms. During each storm, for a given discharge the amplitude of seismic waves are on average two times greater on the rising limb of the storm than on the falling limb, in both aggradational and erosional events. Clockwise hysteresis in both aggradational and erosional events implies that water turbulence, alone, is not the source of the seismic waves. If seismic wave amplitude correlates linearly with the flux of bedload, this implies a roughly two‐fold decrease in transport efficiency over the time‐scale of days during individual storms. The observed change in transport efficiency can plausibly be explained by the disturbance of bed armor during storms and subsequent reformation during the waning stages. This data highlights the potential for fluvial seismology to reveal the dynamics of bedload transport. Citation: Hsu, L., N. J. Finnegan, and E. E. Brodsky (2011), A seismic signature of river bedload transport during storm events, Geophys. Res. Lett., 38, L13407, doi:10.1029/2011GL047759. 1. Introduction [ 2 ] Gravel and cobble transport by rivers governs channel change, and is therefore of great importance to geomor- phologists and river engineers. The most commonly used approaches to estimating bedload transport rates are empir- ically calibrated relationships based on flume experiments [e.g., Meyer‐Peter and Muller, 1948; Wilcock and Crowe, 2003]. However, it is difficult to assess how effectively such relationships predict transport rates during extreme events in large rivers because quantitative measures of bed- load transport are labor intensive and, at high flows, often dangerous to obtain. For logistical reasons, most bedload studies have been carried out in small mountain streams. [ 3 ] Particle impacts on the river bed transfer momentum, which in turn generates elastic (seismic) waves. Therefore, seismology can potentially constrain bedload transport rates in rivers. High frequency (>1 Hz) seismic waves have been Department of Earth and Planetary Sciences, University of California, Santa Cruz, California, USA. Now at Lamont-Doherty Earth Observatory, Palisades, New York, USA. Copyright 2011 by the American Geophysical Union. 0094‐8276/11/2011GL047759 used to study earth surface processes such as ocean waves [Adams et al., 2002], landslides [Favreau et al., 2010], rockfalls [Deparis et al., 2008], debris flows [LaHusen, 2005; Suwa et al., 2003; Burtin et al., 2009], and snow avalanches [Vilajosana et al., 2007; Cole et al., 2009]. In all of these cases, remote monitoring is possible because geo- phones or seismometers capture ground vibrations caused by the surface process. A few studies have used seismo- logical data to study fluvial processes [e.g., Govi et al., 1993], and recent work showed that high frequency seis- mic waves may provide a way to monitor bedload sediment transport in inaccessible rivers in the Himalaya. Burtin et al. [2008] showed that seismic wave amplitude along the Trisuli River in Nepal correlated well with hydrological processes such as daily precipitation and river discharge cycles. In addition, over the year of available data, the seismic ampli- tude was greater for a given river stage at the start of the Indian Monsoon than at the end. This was interpreted as evidence for a bedload source for seismic waves, as opposed to water turbulence, because significant hysteresis is not expected in the relationship between stage and turbulence, whereas hysteresis is common in sediment rating curves. However, in addition to sediment supply, many other factors (e.g., bed armoring, channel geometry) might influence the relationship between river stage and the amplitude of ground vibrations. [ 4 ] In this paper we move closer to a direct connection between bedload transport and seismic wave amplitude by studying one site on the Cho‐Shui River in Taiwan that has experienced repeated, short‐lived, and quantitatively‐ documented typhoons. This dataset combines nearby water level, channel cross‐section, discharge, and seismic ampli- tude measurements from existing infrastructure. A major goal of our work is to determine if a consistent, time‐varying relationship between the amplitude of ground vibrations and river flux holds during multiple events. By examining several events with different durations and intensities, we show that during each storm there is a clear distinction between the seismic amplitudes generated for a given water discharge as the water rises compared to when it falls. Whether the event is aggradational or erosional, there is a greater amplitude of seismic noise at the beginning of the storm in all three typhoon events. This key observation combined with a contrasting observation for smaller subevents and simple hydraulic arguments suggests that bedload transport controls the seismic wave generation. Furthermore, the observed hysteresis is consistent with an increase in the threshold stress for sediment entrainment during each event caused by packing and/or coarsening of the alluvial bed. 2. Site and Methods [ 5 ] The study site on the Cho‐Shui River in Taiwan was selected because it has highly variable water discharge, high L13407 1 of 6

Landscape evolution, valley excavation, and terrace development following abrupt postglacial base-level fall

Many high-latitude fluvial systems are adjusting to base-level changes since the last glaciation. Channels that experienced base-level fall may still be incising, often through glacial diamictons (tills). These tills can be quite competent, behaving more like weak bedrock than unconsolidated sediment, and erode at a fast pace, thus providing a unique opportunity to test models of channel incision and knickpoint migration in transient systems. Here, we integrate light detection and ranging (LiDAR) topography, strath terrace chronology, and numerical modeling to determine knickpoint migration and incision history of the Le Sueur River in central Minnesota, USA. Results indicate that the Le Sueur River is best modeled as a detachment-limited channel, with downstream coarsening related to lag clasts from tills playing a critical factor in longitudinal profile development. The Le Sueur River meanders as it incises, so we coupled the best-fit incision model to a meander model to determine valley excavation history. The excavation history was used to determine a natural background erosion rate, prior to land-use changes associated with settlement and agricultural expansion in the mid-1800s. We compared background fine sediment (silt and clay) erosion rates with historic decadal-average annual suspended loads. Results show that modern fine sediment contributions from sources associated with valley excavation are three times higher than modeled presettlement loads. Recent changes in hydrology associated with land use and climate change have increased flows in rivers, leading to higher sediment loads, not just from field erosion, but from increased bank and bluff erosion in the deeply incised valleys.

Sediment supply, base level, braiding, and bedrock river terrace formation: Arroyo Seco, California, USA

In many settings, rivers alternate between carving wide valley bottoms (straths) and cutting narrow gorges over time, thereby creating longitudinally continuous paired bedrock strath terraces along valleys. Strath terraces are used ubiquitously in geomorphology and tectonics; however, how and why they form remain poorly understood. Here, we focus on Arroyo Seco in the central California Coast Ranges, where we test hypotheses for strath planation and subsequent strath terrace formation. Several lines of evidence indicate that strath planation is triggered by braiding in bedrock channels. In particular, hydraulic modeling reveals that the width of Arroyo Seco’s most recently formed terrace is comparable to the width of currently braided channel reaches. Additionally, a comparison of currently braided reaches to abandoned bedrock meander cutoffs shows that braided channels have valleys that are several times wider than single-thread meandering bedrock channel reaches. Lastly, in locations where the modern channel is currently braided, terraces are poorly preserved, suggesting that evidence for past episodes of braiding, in the form of paired strath terraces, is apparently largely destroyed by subsequent episodes of braiding. Field observations combined with mapping of terrace levels using an objective light detection and ranging (LiDAR)–based terrace identification algorithm reveal that temporal variation in tectonic uplift rate, sea level, and/or alluvial cover along the river cannot explain strath planation and subsequent terrace formation in Arroyo Seco. Rather, our results provide evidence that aggradation and degradation of alluvial sediments downstream of the Reliz Canyon fault result in impulsive base-level forcing of Arroyo Seco’s bedrock channel. Strath abandonment and terrace formation apparently occur as incision into downstream alluvial sediments propagates upstream into bedrock. Braiding and planation of straths, in contrast, occur during intervals of low vertical incision rate associated with downstream aggradation or immediately following pulses of vertical lowering triggered by downstream incision of alluvial sediments.

Accounting for Atmospheric Delays in InSAR Data in a Search for Long-Wavelength Deformation in South America

InSAR has been successfully used to observe the deformation of the Earths surface from many processes, but mostly dealing with relatively large signals (>;1 cm) over short wavelengths (<; 100 km). We use interferometric synthetic aperture radar (InSAR) data from two orbital tracks in northern Chile to study the feasibility of imaging the broad interseismic ground deformation signal from the Nazca Plate subduction. In order to measure ~1.5 cm/year of ground motion across ~1000 km of satellite track length due to interseismic loading of the subduction interface, the atmospheric contribution cannot be ignored. We attempt to remove the atmospheric signal using global weather models and by estimating atmospheric parameters directly from the InSAR data. Due to the poor temporal and spatial resolutions of the weather model, this method fails to produce reliable results. The empirical model reduces the phase variance in the interferograms but leaves a residual signal that continues to mask the interseismic signal, which demonstrates the importance of carefully applying corrections to the data as they can significantly affect any interpretation that is based on the corrected observations. Although the methods presented here are not suited for removing all atmospheric path delays, this paper does provide suggestions about improvements that can be made to corrective techniques. Methods that should be further developed are the following: 1) corrections with independent and direct observations of atmospheric properties, e.g., continuous GPS or satellite observations (e.g., the MERIS sensor); 2) improvements using empirical corrections, either in conjunction with a deformation model or constrained by real atmospheric structures; and 3) further work with high resolution and improved weather models.

Landscape Evolution in South-Central Minnesota and the Role of Geomorphic History on Modern Erosional Processes

The Minnesota River Valley was carved during catastrophic drainage of glacial Lake Agassiz at the end of the late Pleistocene. The ensuing base-level drop on tributaries created knickpoints that excavated deep valleys as they migrated upstream. A sediment budget compiled in one of these tributaries, the Le Sueur River, shows that these deep valleys are now the primary source of sediment to the Minnesota River. To compare modern sediment loads with pre-European settlement erosion rates, we analyzed incision history using fluvial terrace ages to constrain a valley incision model. Results indicate that even though GSA Today, v. 21. no. 9, doi: 10.1130/G121A.1.

Magnitude and duration of surface uplift above the Socorro magma body

We use interferometric synthetic aperture radar and geomorphic data to constrain the magnitude and duration of uplift driven by the magma body beneath Socorro, New Mexico, United States. Interferometry spanning 1992–2006 confirms uplift of the Socorro magma body at a rate of ~2.5 mm/yr. However, we find no clear evidence for volcanic uplift after an examination of three rivers (Rio Salado, Rio Puerco, Rio Grande), and two terraces (Llano de Manzano, Llano de Albuquerque) crossing the Socorro magma body. Our geomorphic measurements permit at most 25–50 m of cumulative surface uplift above the Socorro magma body since the middle Pleistocene, but require no long-term uplift. Given previously articulated thermal arguments for the Socorro magma body, we therefore suggest either a recent (within the last few centuries) initiation of uplift at Socorro, or that long-term uplift and subsidence have been essentially equal.

EROSIONAL RESPONSE TO NORTHWARD-PROPAGATING CRUSTAL THICKENING IN THE COASTAL RANGES OF THE U.S. PACIFIC NORTHWEST

We measured basin-scale erosion rates, using cosmogenic 10Be concentrations in quartz, from fluvial sediment in rivers draining the coastal mountain ranges of the U.S. Pacific Northwest between 40° and 47° N. Apparent erosion rates are 0.1 to 0.2 mm yr−1 throughout the Oregon Coast Ranges north of 43° N, and increase to the south to 0.6 to 1.1 mm yr−1 in the northern California coast ranges near 40° N. We propose that these observations display the erosional response to northward-migrating crustal thickening associated with subduction of the Mendocino Triple Junction. North-south variations in erosion rate, range elevation, and metrics of landscape relief and steepness are consistent with the hypotheses that i) their primary cause is northward-migrating crustal thickening; ii) erosion rates are strongly controlled by topographic relief and weakly, if at all, controlled by climate; and iii) the dependence of erosion on relief is nonlinear and obeys a threshold-relief relationship.