Debris cover and the thinning of Kennicott Glacier , Alaska , Part C : feedbacks between melt , ice dynamics , and surface processes

The mass balance of many valley glaciers is enhanced by the presence of melt hotspots within otherwise continuous debris cover. We assess the effect of debris, melt hotspots, and ice dynamics on the thinning of Kennicott Glacier in three companion papers. In Part A we report in situ measurements from the debris-covered tongue. In Part B, we develop a method to delineate ice cliffs using high-resolution imagery and produce distributed mass balance estimates. Here in Part C we describe feedbacks controlling rapid thinning under thick debris. Despite the extreme abundance of ice cliffs on Kennicott Glacier, average melt rates are strongly suppressed downglacier due to thick debris. The estimated melt pattern therefore appears to reflect Østrem’s curve (the debris thickness-melt relationship). As Kennicott Glacier has thinned over the last century Østrem’s curve has manifested itself in two process domains on the glacier surface. The portion of the glacier affected by the upper-limb of Østrem’s curve corresponds to high melt, melt gradients, and ice dynamics, as well as high ice cliff and stream occurrence. The portion of the glacier affected by the lower-limb of Østrem’s curve corresponds to low melt, melt gradients, and ice dynamics, as well as high ice cliff and stream occurrence. The upglacier end of the zone of maximum thinning on Kennicott Glacier occurs at the boundary between these process domains and the bend in Østrem’s curve. The expansion of debris upglacier appears to be linked to changes in the surface velocity pattern through time. In response to climate warming, debris itself may therefore control where rapid thinning occurs on debris-covered glaciers. Ice cliffs are most abundant at the upglacier end of the zone of maximum thinning.

More general comments 1. This paper is the last (part c) of a series of 3 papers, and one could always ask if not all parts should have been integrated into one paper. I admit that some repetition (in the description of some the data sets for example) is unavoidable, but for me part C works very well as a stand-alone paper and has a very clear own focus on the dynamic feedbacks and interactions and more than enough conclusive results for a stand-alone paper. I have to note that I only briefly looked into the other two papers (part A and B) but it was clear that there the main aim and focus of the otehr parts (A and B) were substantially different and in my view justified as separate papers. Moreover I believe that the main messages and findings of the three papers come in separate papers probably better across than in one huge one.
Thank you kindly. If we do not reply to the comment below we will enact the change in Part C.
2. Abstract focus: somewhat related to the 3-part paper thing, when reading the abstract I got the impression that the main focus of the paper on the feedbacks and interrelations between processes to explain the thinning pattern is rather thinly represented within the abstract (last 3 lines, and little on feedbacks but rather on correlations) and the results used in this papers but from part A and B get too much space in the abstract. A better balance and more focus on the feedbacks and findings of THIS part C would be useful.
We will follow this advice.
3. Difference in time periods of datasets: One potential criticism of the analysis and conclusions one could have is that the thinning-data represents an average over several decades whereas the velocities and surface features, debris extent etc are the 'now' situation. I myself do not really think this is really a big issue but some more explanation and justification for this maybe useful.
We have additional dh/dt data that covers the in situ measurement time period. 4. Literature: With regard to influence/link of ice dynamics to thinning, debris cover and ice cliffs (e.g. explaining anomalous thinning) the paper by Banjeree ( Thanks we will include these citations. Minor/specific comments p. 1 Line 15: the term 'melt hotspots' is here not really clear maybe specify a bit more what it is ('melt hotspots such as ice cliffs or channels') p 1 lines 23-27: maybe make clearer what of these results from this part C paper and what is from earlier (or really focus on part C part). P 1 line 24-25: high melt and HIGH melt gradients? here and also on next line it is not so clear to me what you mean by 'melt gradients' here, 'spatial gradients in melt' along flow, gradients in melt with regard to changing debris thickness. . .. be clearer. P1 line 31: a brief explanation why ice cliffs are most abundant at the upglacier end maybe useful here (I think you have some idea about this or am I wrong?). P 2 line 50: I think 'surface' uplift is here not quite correct, ice emergence is the relative movement to the surface or particle uplift against the surface, so maybe 'ice' uplift is more appropriate.
Thanks for clarifying this for us. P 2 line 56-57: '. . .will facilitate the INTERPRETATION AND prediction of. . .' P 2 line 62-63: importantly in part C you not just present data on ice dynamics and supraglacial streams but crucially in part C these data and all components of the mass conservation equation (thinning, flluy divergence. . .) are analysed for relation and feedbacks between them. Also say this here, as it is the backbone and most exiting part of this part C.
Thanks for this.
p. 3 lines 73-79: is this paragraph on the water pressure variations and sliding really needed? Maybe just summarize it in one sentence in the section 2.1 or 3.1 on the velocity data.
We will clarify this and make the connections more clear. We will.  We will change this. Fig. 7: here an elevation threshold/bands are used to summarize/group the sinuosity data, but am I right that these are both above the ZMT and in the upper limb of the oestroem curve. This maybe useful to be explained in the caption.
We will change this. Fig. 10. I found this figure rather difficult to read, there is a lot of information and detail and I initially expected from this schematic to better get the big picture. Maybe I just expected the wrong thing and the colors (blue or red) were not so clear to me and I wondered if it really helped me a lot. If I see it as complete documentation of all different relations and feed backs it is maybe fine, but then maybe it should be phrased as such. More importantly, in the caption the colors red and blue refer to positive effects or negative effects but it is not so obvious to me what you mean by positive and negative. Does this refer to positive and negative FEEDBACKS (self enhancing/reducing) or positive/negative from a glacier health (negative mass loss, reduced speed,. . ..). should be clarified.
We will clarify this figure or remove it.