Publication Date:
2023-01-20
Description:
River‐valley morphology preserves information on tectonic and climatic conditions that shape landscapes. Observations suggest that river discharge and valley‐wall lithology are the main controls on valley width. Yet, current models based on these observations fail to explain the full range of cross‐sectional valley shapes in nature, suggesting hitherto unquantified controls on valley width. In particular, current models cannot explain the existence of paired terrace sequences that form under cyclic climate forcing. Paired river terraces are staircases of abandoned floodplains on both valley sides, and hence preserve past valley widths. Their formation requires alternating phases of predominantly river incision and predominantly lateral planation, plus progressive valley narrowing. While cyclic Quaternary climate changes can explain shifts between incision and lateral erosion, the driving mechanism of valley narrowing is unknown. Here, we extract valley geometries from climatically formed, alluvial river‐terrace sequences and show that across our dataset, the total cumulative terrace height (here: total valley height) explains 90%–99% of the variance in valley width at the terrace sites. This finding suggests that valley height, or a parameter that scales linearly with valley height, controls valley width in addition to river discharge and lithology. To explain this valley‐width‐height relationship, we reformulate existing valley‐width models and suggest that, when adjusting to new boundary conditions, alluvial valleys evolve to a width at which sediment removal from valley walls matches lateral sediment supply from hillslope erosion. Such a hillslope‐channel coupling is not captured in current valley‐evolution models. Our model can explain the existence of paired terrace sequences under cyclic climate forcing and relates valley width to measurable field parameters. Therefore, it facilitates the reconstruction of past climatic and tectonic conditions from valley topography.
Description:
Plain Language Summary:
Little is known on how valleys widen and what sets their width. Therefore, it remains difficult to model the wealth of valley geometries that occur in nature and to predict how valleys adjust to environmental changes. Paired river terraces are staircases of abandoned valley floors that preserve valley widths of the past. The formation of river‐terrace sequences requires changes between vertical river incision and lateral river erosion of valley walls. Moreover, to preserve terraces on both sides of the river, the valley has to narrow over time. While cyclic climate changes during the Quaternary can explain the alternations between vertical incision and lateral erosion, they cannot explain why those valleys narrow. Here we investigate past valley geometries in paired, climatically formed river terraces. We find a negative linear relationship between valley width and valley height. We propose that this relationship reflects a balance between sediment that is moved from hillslopes into the channel and the capacity of the river to remove this sediment. Higher valley walls contribute more sediment that protects the wall from further widening. By including this hillslope‐erosion term, valley‐formation models can reproduce paired river terraces, and allow us to work toward “reading” climatic conditions from valley geometries.
Description:
Key Points:
Valley width in alluvial terraces is inversely proportional to valley height.
We suggest sediment supply from river‐independent hillslope erosion limits valley width.
The coupling of hillslopes and river channels demands revision of current valley‐evolution models.
Description:
EC H2020 PRIORITY “Excellent science” H2020 Marie Skłodowska‐Curie Actions
http://dx.doi.org/10.13039/100010665
Description:
https://doi.org/10.5880/fidgeo.2022.021
Keywords:
ddc:551.3
;
valley width
;
river terraces
Language:
English
Type:
doc-type:article
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