|Re-review of Chambers et al|
The authors have made an effort to address the referees’ original set of comments and largely the changes have achieved this. The inclusion of Fig 7, in particular, provides greater context for the more speculative elements of this paper regarding the presence of a continuous active hydrological channel along the valley identified in earlier studies. There is, however, still a considerable amount of speculative material and sweeping statements that would benefit from improvement.
Title. The authors have argued to keep the original title (with some small changes) rather than focusing on the impact of the modified bed topography on water routing and ice motion. They have also decided to retain the use of the term “subglacial river”. The former is OK but the latter I find problematic. There are many papers on subglacial hydrology and several nice reviews. One of the most recent focusing on Greenland is [Davison et al., 2019]. This and all the other articles I am aware of use accepted glaciological notation and definitions. I can find no reference to “subglacial rivers” and, given the extremely speculative nature of the paper, it seems entirely unjustified to introduce a new term or concept with no theoretical or empirical foundation. The authors also refer to “braided rivers” (l27 p 2). This is also a term that is new to the field of subglacial hydrology. The authors provide no basis for introducing these new concepts in subglacial hydrology. The justification given on p2 is entirely inadequate and inappropriate. The section in the discussion, already commented on in the previous reviews, simply rehearses in a qualitative way established concepts in subglacial hydrology. The term “braided river” is not discussed anywhere. If the authors wish to introduce new terminology into the discipline they need to provide a justification. None exists and they should use accepted terminology to avoid confusion, ambiguity and misleading readers.
They should replace terms such as “subglacial river channel” with “subglacial water channel” and remove reference to a river from the m/s. The title would then replace “subglacial river” with “active subglacial water channel”
P2, l11, l20 and elswhere. Data is plural.
P2, l16. The authors claim there is a continuity of the signature of the valley on the ice sheet surface where the bed topo has a valley. This requires at the minimum a citation to a study that demonstrates this although I do not believe such a study exists. In fact, Fig 2 of [Ekholm et al., 1998] suggest the exact opposite. The surface expression of the “valley” is intermittent and aligns well with the bed topo valley locations. In other words, the surface expression suggests the contrary: that the “valley” may not be continuous, or at least, it’s depth and width may not be sufficient to produce a unique surface expression everywhere. Further, fig 4 from the same paper indicates that the depth of the valley, where observed, is highly variable over short distances: 100 m deep and narrow in fig c and 200 m deep and twice the width in Fig b around 30 km upstream. Assuming a fixed depth of 400 m will, by default, result in water being routed along the valley because it is approximately aligned with the surface slope.
The authors do investigate the sensitivity to assumed depth in Fig A3 and the water routing, not surprisingly, is sensitive to this. The IPR data indicates, however, a variable width and depth valley. This is also confirmed in Fig 2 of [Bamber et al., 2013] that suggests a valley that ranges in depth from ~500 m to <100 m. It would be useful for the authors to acknowledge the idealised nature of their “thought” experiment and that it is not designed to replicate the observed characteristics of the valley and is just that “a thought expt”, which is fine as long as the m/s reflects this appropriately.
P11, l29-30 “However, since water has already been detected in the valley (Ekholm, 1998). “ This is too firm a statement and needs to be rephrased to something more accurate such as “since water has been inferred to be present from IPR data in a limited number of locations”
Bamber, J. L., M. J. Siegert, J. A. Griggs, S. J. Marshall, and G. Spada (2013), Paleofluvial Mega-Canyon Beneath the Central Greenland Ice Sheet, Science, 341(6149), 997-999.
Davison, B. J., A. J. Sole, S. J. Livingstone, T. R. Cowton, and P. W. Nienow (2019), The Influence of Hydrology on the Dynamics of Land-Terminating Sectors of the Greenland Ice Sheet, Frontiers in Earth Science, 7(10), doi:10.3389/feart.2019.00010.
Ekholm, S., K. Keller, J. L. Bamber, and S. P. Gogineni (1998), Unusual surface morphology from digital elevation models of the Greenland ice sheet, Geophys. Res. Lett., 25(19), 3623-3626.