the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Nov 2024
Controls on fluvial grain sizes in post-glacial landscapes
Abstract. The grain sizes of sediments in channels have been linked to landscape characteristics, such as flow distance from headwaters, topographic relief, lithology and climate, in landscapes with little past or present glacial influence. Few studies have explored the controls on sediment characteristics in formerly glaciated landscapes. In this study, we document river surface grain sizes at 279 localities across Scotland. We collect photographs of gravel bars through a citizen science survey, Scotland's Big Sediment Survey. Grain sizes distributions are extracted from the photographs using both manual and automated techniques. We investigate whether grain sizes can be correlated and predicted from environmental variables (e.g., basin slope, flow distance from headwaters) through Spearman's correlation statistics and random forest regression modelling. In contrast to other studies that have primarily focused on non-glaciated landscapes, we find no apparent controls on surface grain sizes in channels across Scotland. Specifically, we find no significant Spearman's relationships between d84 and environmental variables; the strongest relationship was found between d84 and average basin aridity with a weak r2 value of 0.29. We also find that the predictability of our random forest model is poor and only captures 22 % of the variance of d84. We find no correlation between grain size and flow competence, which suggests that sediment is both transport-limited and supply-limited. We propose that Scotland's post-glacial legacy drives the lack of sedimentological trends documented in this study, and that changes in landscape morphology and sediment sources caused by glacial processes lead to a complete decoupling between fluvial sediment grain size and environmental variables. This interpretation aligns with other studies that have highlighted the ongoing role of the post-glacial legacy on landscape evolution in tectonically quiescent terrains, both in Scotland and globally. Our results suggest that fluvial sediment grain size cannot be predicted by a global model based on environmental variables in post-glacial landscapes.
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RC1: 'Comment on egusphere-2024-3084', Katie Whitbread, 12 Jan 2025
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This paper presents a broad analysis of grain size in streams in postglacial settings which builds on previous studies in non-glaciated landscapes that have been conducted using random forest and Spearman correlations (Snelder, 2011 and others). These methods are employed by the authors in a novel landscape setting and with an alternative approach use to data capture - a citizen science survey. The paper frames a clear hypothesis and is well structured and clearly written. I consider the paper will provide a valuable contribution to the field following some revision. I have three main points which I think should be addressed to enhance the work for publication.
As outlined in the main points below, I feel the paper would benefit from further discussion of the limitations of the data captured by the citizen science survey approach, including the distribution of sites and the representativeness of the images of sediment characteristics at the reach scale. Additionally, (and if supported by the data), I think more could be made of the sites where multiple data points are available for a single stream, particularly to illustrate the model for sediment delivery established for post-glacial settings.
A more substantial point relates to the erodibility metric used, and I believe further consideration of the formulation of this metric may be required to ensure it adequately reflects the geological variability in Scotland. Although I suggest the metric needs revision, I don’t believe changes will alter the main conclusion of the paper. I look forward to seeing the final work in print.
Best wishes,
Katie WhitbreadMain points
1. Data distribution and representativeness:
The sites are quite clustered and some discussion of potential impact of clustering bias on the outcomes is needed. To what extent does the distribution of sites cover a reasonable range of values of your key parameters (erodibility, steepness etc.)? Are very big, or very small channels undersampled? Could you include some plots in the supplementary information?
Note that the distribution of data in Snelder et al. (2011) is a well-spread sampling pattern at the national scale which is pretty much ideal, so it’s important to consider the effect of clustering in relation to the lack of correlation in your study.
In terms of representativeness of the sample, grain size may vary locally within reaches, e.g. between riffles and pools, inner channels and channel bars, even between adjacent bars. The Snelder (2011) study describes a process of visual assessment of the geomorphic components of a reach and areal proportions of different grain-size deposits, followed by selection of representative points for sampling. Are there ways you can assess the representativeness of the sample (in the context of the reach morphology)?
The paper would benefit from further consideration of the distribution and representativeness of the data in the discussion as part of a more extended treatment of the limitations of the crowd-sourcing approach.2. Analysis of single-stream sample groups
The treatment of sites where multiple samples are available for the same stream is very limited. It would be interesting if more could be made of this data – for example a fuller treatment of the data for the River Feshie shown in figure 6 (b) to assess whether there are localised spatial trends and points where disruption occurs due to sediment inputs. Is there potential to include e.g. long profiles where data and key local features of influence could be plotted data could be projected on to a long profile? This could help to illustrate the spatial model in figure 7 and bolster the conclusion by demonstrating it operating through the local dynamics in the study area.3. The erodibility metric
Metamorphic grade is encompassed by the lithology described on BGS maps (e.g. a gneiss is high metamorphic grade by definition, and a metamorphosed sandstone is either a “metasandstone” or a “wacke”). I’m therefore not convinced of the relevance of combining separate metrics reflecting lithology and metamorphic grade when using the BGS bedrock map. The metamorphic grade seems redundant. Also, what is the estimate of rock strength (LL) based on? Has a strength dataset been used?The classification shown in the erodibility map in supplementary Figure S2 is different to what I would expect, and I think this points to the need for revision of the metric.
• The moderate erodibility estimated for the Ordovician-Silurian age metasandstones (“wackes”) across the Southern Uplands appears similar to the Devonian-aged Caithness Flagstone (an unmetamorphosed lacustrine siltstone/fine sandstone sequence) in the far northeast, and to the Coal Measures of Carboniferous age in the Midland Valley. I’d expect the Southern Uplands ‘wackes’ to have lower erodibility.
• The highest erodibility units (yellow) seem to be associated with small outcrops of Early to Middle Devonian conglomerates around the fringes of the Moray Firth and with units that occur along the Highland Boundary Fault zone and Southern Uplands Fault. It is not clear why these specific Devonian conglomerates should be more erodible than other Devonian units including conglomerates, sandstone and lacustrine siltstone and mudstone which are estimated with medium erodibility.
• I am unsure what would be giving rise to the high erodibility values along the Highland Boundary Fault – perhaps the Highland Border Complex, but these strata are metamorphosed.
• I would expect the highest erodibility in strata in the unmetamorphosed Permian sandstones which occur in basins within the Southern Uplands and small exposures of Triassic/Jurassic/Permian strata around the Moray Firth Coast - but currently these seem to have intermediate erodibility values.Finally – this approach to characterising rock erodibility was developed to configure models of river incision into rock rather than as a control on grain-size distributions supplied to channels. It would be worth noting, in the discussion at least, that grain-sizes supplied to channels are significantly influenced by the nature of discontinuities (joints and faults) – see overview in Sklar (2024). Whilst in general terms, lithology/rock strength influences fracturing and is therefore somewhat included in your method, fracture density can be very locally variable (e.g. Neely & DiBiase, 2020; Whitbread et al., 2024) and the degree to which this may influence the supply of material to your sites isn’t known. There has been a fair amount of recent work on grain-size distributions on hillslopes and I think it is important to note the emerging literature in this area and consider it in relation to your conclusions.
Specific comments:
Figure 1: Increasing the size of the maps and photos would be helpful – these are very small in the preprint. Add labels showing key locations mentioned in the text would also be useful (see comment below about image 3). If addressing the point above about the use of multiple points on key streams, could you highlight grouped data for key streams on this plot? Or perhaps show those more clearly on Figure 3). Use of a hillshade terrain model as the base map may also help illustrate the range of topographic settings associated with the data distribution.
Figure 2: the segmentation of the photograph in c) is very difficult to see – increasing the photo size and upping the contrast of the lines would be helpful.
Figure 3: Labelling is included on the map, but not very clearly – increasing the image size and using lines to show the association of the label and relevant points more clearly would be helpful.
Figure 7: Could you include an illustration of this model from one or more of your catchments where you have multiple points on the same stream? See notes on the text below. (But if you leave the figure as is, can you increase the size of the photos as they are too small to see clearly.)
Supplementary figures S1 and S2: Increase the size of the maps
Line 186 / Table 1: What scale and version was the bedrock geological map used? E.g. was it 1:50,000 Bedrock v8.
Line 198 / Table 1: What scale and version was the superficial geological map used?
Line 242 / Equation 6: The inclusion of Q and W in equation 6 means that it isn’t just rearranged from equation 5. Could you explain the inclusion or cite a relevant paper?
Line 264: There is very cursory treatment of the data for rivers with multiple samples. How many rivers had multiple samples? How many samples and over what range of drainage areas? Could you show the variation of D84 and potentially other variables on a long profile of the stream? This may be particularly useful if it helps illustrate your model in Figure 7.
Line 291 / Figure 6 (b): See main point 2 above – is there more in this data for the Feshie? It seems like there are perhaps two clusters of data, one with a trend and one without. Are there parts of the catchment with localised trends in flow competence, vs. parts disrupted by sediment inputs? Could you use local relationships in the Feshie or other streams to illustrate your model in Figure 7?
Lines 345-352: The discussion of the citizen science approach should also address limitations of the approach and consider how these could be mitigated in future studies. As noted above, the clustered distribution of sampling sites is a key limitation – I think you should include some discussion of the potential impact of this on the outcome of your study.
References:
Neely, A.B. and DiBiase, R.A., 2020. Drainage area, bedrock fracture spacing, and weathering controls on landscape‐scale patterns in surface sediment grain size. Journal of Geophysical Research: Earth Surface, 125(10), p.e2020JF005560.
Sklar, L.S., 2024. Grain Size in Landscapes. Annual Review of Earth and Planetary Sciences, 52.
Whitbread, K., Thomas, C. and Finlayson, A., 2024. The influence of bedrock faulting and fracturing on sediment availability and Quaternary slope systems, Talla, Southern Uplands, Scotland, UK. Proceedings of the Geologists' Association, 135(1), pp.61-77.Citation: https://doi.org/10.5194/egusphere-2024-3084-RC1
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