Mountain landscapes have dynamic climates that, together with tectonic processes, influence their topographic evolution. Spatial and temporal variations in rainfall are ubiquitous in these settings as orographic precipitation patterns evolve with climate change and topography. Despite important implications such changes have for river incision, their influence is understudied. Here, we investigate how changes in rainfall pattern should affect both the steady state form and transient evolution of river profiles at the catchment scale using the stream power model. We find that spatially varied rainfall patterns can complicate steady state relationships between mean rainfall, channel steepness and fluvial relief, depending on where rainfall is concentrated in catchments, and lead to unexpected transient behavior if they are neglected. Specifically, changes in rainfall pattern cause multi‐stage transient responses that differ from responses to uniform changes in rainfall. Disparate responses by rivers that experience different rainfall conditions, particularly trunk and tributary rivers, are also an important factor in understanding catchment‐wide responses to climate change. Accounting for such disparities in sampling strategies and topographic analyses may, therefore, be vital for detecting and quantifying climate's role in landscape evolution. Lastly, we show how explicitly accounting for rainfall patterns in channel steepness indices, and thus spatial variations in erosional efficiency, may advance understanding of landscape sensitivity to climate. These results have important implications for detecting transient responses to changes in rainfall pattern (and more broadly climate), interpretation of morphometrics in steady state and transient landscapes, and quantifying the sensitivity of landscapes and erosion rates to climate.
This content will become publicly available on September 1, 2024
Bedrock rivers are the pacesetters of landscape evolution in uplifting fluvial landscapes. Water discharge variability and sediment transport are important factors influencing bedrock river processes. However, little work has focused on the sensitivity of hillslope sediment supply to precipitation events and its implications on river evolution in tectonically active landscapes. We model the temporal variability of water discharge and the sensitivity of sediment supply to precipitation events as rivers evolve to equilibrium over 106model years. We explore how coupling sediment supply sensitivity with discharge variability influences rates and timing of river incision across climate regimes. We find that sediment supply sensitivity strongly impacts which water discharge events are the most important in driving river incision and modulates channel morphology. High sediment supply sensitivity focuses sediment delivery into the largest river discharge events, decreasing rates of bedrock incision during floods by orders of magnitude as rivers are inundated with new sediment that buries bedrock. The results show that the use of river incision models in which incision rates increase monotonically with increasing river discharge may not accurately capture bedrock river dynamics in all landscapes, particularly in steep landslide prone landscapes. From our modeling results, we hypothesize the presence of an upper discharge threshold for river incision at which storms transition from being incisional to depositional. Our work illustrates that sediment supply sensitivity must be accounted for to predict river evolution in dynamic landscapes. Our results have important implications for interpreting and predicting climatic and tectonic controls on landscape morphology and evolution.
more » « less- Award ID(s):
- 1727736
- NSF-PAR ID:
- 10480540
- Publisher / Repository:
- Journal of Geophysical Research-Earth Surface
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Earth Surface
- Volume:
- 128
- Issue:
- 9
- ISSN:
- 2169-9003
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
more » « less
Abstract Sediment grain size links sediment production, weathering, and fining from fractured bedrock on hillslopes to river incision and landscape relief. Yet models of sediment grain size delivery to rivers remain unconstrained due to a scarcity of field data. We analyzed how bedrock fracture spacing and hillslope weathering influence landscape‐scale patterns in surface sediment grain size across gradients of erosion rate and hillslope bedrock exposure in the San Gabriel Mountains (SGM) and northern San Jacinto Mountains (NSJM) of California, USA. Using ground‐based structure‐from‐motion photogrammetry models of 50 bedrock cliffs, we showed that fracture density is ~5 times higher in the SGM than the NSJM. 274 point‐count‐surveys of surface sediment grain size measured in the field and from imagery show a drainage area control on sediment grain size, with systematic downslope coarsening on hillslopes and in headwater‐colluvial channels transitioning to downstream fining in fluvial channels. In contrast to prior work and predictions from a hillslope weathering model, grain size does not increase smoothly with increasing erosion rate. For soil‐mantled landscapes, sediment grain size increases with increasing erosion rates; however, once bare bedrock emerges on hillslopes, sediment grain size in both the NSJM and SGM becomes insensitive to further increases in erosion rate and hillslope bedrock exposure, and instead reflects fracture spacing contrasts between the NSJM and SGM. We interpret this threshold behavior to emerge in steep landscapes due to efficient delivery of coarse sediment from bedrock hillslopes to channels and the relative immobility of coarse sediment in fluvial channels.
-
more » « less
Incising rivers may be confined by low-slope, erodible hillslopes or steep, resistant sidewalls. In the latter case, the system forms a canyon. We present a morphodynamic model that includes the essential elements of a canyon incising into a plateau, including 1) abrasion-driven channel incision, 2) migration of a canyon-head knickpoint, 3) sediment feed from an alluvial channel upstream of the knickpoint, and 4) production of sediment by sidewall collapse. We calculate incision in terms of collision of clasts with the bed. We calculate knickpoint migration using a moving-boundary formulation that allows a slope discontinuity where the channel head meets an alluvial plateau feeder channel. Rather than modeling sidewall collapse events, we model long-term behavior using a constant sidewall slope as the channel incises. Our morphodynamic model specifically applies to canyon, rather than river–hillslope evolution. We implement it for Rainbow Canyon, CA. Salient results are as follows: 1) Sediment supply from collapsing canyon sidewalls can be substantially larger than that supplied from the feeder channel on the plateau. 2) For any given quasi-equilibrium canyon bedrock slope, two conjugate slopes are possible for the alluvial channel upstream, with the lower of the two corresponding to a substantially lower knickpoint migration rate and higher preservation potential. 3) Knickpoint migration occurs at a substantially faster time scale than regrading of the bedrock channel itself, underlying the significance of disequilibrium processes. Although implemented for constant climactic conditions, the model warrants extension to long-term climate variation.
-
more » « less
Abstract Coastal rivers that build deltas undergo repeated avulsion events—that is, abrupt changes in river course—which we need to understand to predict land building and flood hazards in coastal landscapes. Climate change can impact water discharge, flood frequency, sediment supply, and sea level, all of which could impact avulsion location and frequency. Here we present results from quasi‐2D morphodynamic simulations of repeated delta‐lobe construction and avulsion to explore how avulsion location and frequency are affected by changes in relative sea level, sediment supply, and flood regime. Model results indicate that relative sea‐level rise drives more frequent avulsions that occur at a distance from the shoreline set by backwater hydrodynamics. Reducing the sediment supply relative to transport capacity has little impact on deltaic avulsions, because, despite incision in the upstream trunk channel, deltas can still aggrade as a result of progradation. However, increasing the sediment supply relative to transport capacity can shift avulsions upstream of the backwater zone because aggradation in the trunk channel outpaces progradation‐induced delta aggradation. Increasing frequency of overbank floods causes less frequent avulsions because floods scour the riverbed within the backwater zone, slowing net aggradation rates. Results provide a framework to assess upstream and downstream controls on avulsion patterns over glacial‐interglacial cycles, and the impact of land use and anthropogenic climate change on deltas.
-
more » « less
Abstract Wide bedrock valleys and their genetic descendants, strath terraces, can serve as morphological records of past climate that reflect river discharge and sediment load during periods of valley widening. Understanding how changes in sediment load and water discharge create such distinct morphological features is limited by a lack of robust understanding of the specific processes of bedrock valley widening. We present results from the first set of flume experiments specifically devoted to exploring the conditions necessary to create wide bedrock valleys and how bedrock valleys develop through time. We ran six experiments in a weak bedrock substrate representing valley widening in an easily erodible bedrock, with differing amounts of water discharge, sediment load and base level fall. We evaluated valley width, valley wall height, channel mobility, lateral and vertical bed incision and sediment cover on the bed to explore the conditions necessary for the development of wide bedrock valleys and better understand the processes that affect valley widening rates. The results of the experiments show that wide bedrock valleys developed slowly and only under long periods of high sediment conditions, while vertical incision occurred much faster and was easily induced under different forcing mechanisms. We found that high sediment flux, enough to cover the channel bed, was a necessary condition for substantial valley widening. However, sediment cover on the bed was not by itself a sufficient condition to create wide bedrock valleys in our experiments; other factors were also required, particularly mobile channels within the valleys and some channel curvature to induce lateral undercutting. The results from this set of experiments suggest that the creation of wide bedrock valleys has several necessary conditions that must be met, and the development of a wide bedrock valley can be disrupted by slight changes in one of these necessary conditions.
