Experimental Study of River Incision: Discussion and Reply Discussion

Experimental study of river incision: Discussion and reply

Discussion

G. H. DURY Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, Wisconsin 53706

While introducing their experiments on river incision into bed existing streams and ancestral larger streams have proved equally rock, Shepherd and Schumm (1974) claimed that the morphologic capable of developing pool-and-riffle sequences in bed rock. details of interfaces between alluvium and bed rock are essentially Although far more abundant evidence would, of course, be wel- unknown. They referred to highs and lows shaped in bed rock as come, it seems reasonable to suggest that the morphologic details possible analogues of the riffles and pools of alluvial rivers but re- of the alluvium/bedrock interface beneath underfit streams are far garded the longitudinal profiles of bedrock streams as effectively from being essentially unknown. On the contrary, those details unstudied (see their p. 261). The clear implication is that interfaces have been deliberately and extensively studied. Similarly, it would and profiles have not been deliberately investigated. Both here and appear that somewhat more than a little is known of the longitudi- in their review of opinions about the inheritance or noninheritance nal profiles of bedrock streams. Australian serials, especially those of incised meanders (266—267), they overlooked a considerable with a geographical title, might perhaps be regarded as not particu- body of published research and commentary. larly accessible sources, except that Schumm was attached to the The question of the possible inheritance of incised meanders Department of Geography in the University of Sydney, Australia, at from an ancient flood plain at high level has been debated over the about the time when the Colo and Hawkesbury Rivers were being years, the debate involving numbers of workers on the European investigated from that department as a base. The results of the ini- mainland (Dury, 1954; 1964a, p. A5). A deliberate attempt to tial subsurface investigation of an underfit stream are cited in qualify, and subsequently to quantify, the relationship between rate Leopold and others (1964), a work which is presumably familiar to of downcutting and rate of enlargement of meander bends (Dury, all fluvial morphologists. A general statement appears in Chorley 1964a, Fig. 18 and accompanying text; 1964b, Fig. 29 and accom- (1969), to which Schumm is a contributor. It is disconcerting, to panying text; 1969) anticipates the conclusion that the relationship say the least, to find that Shepherd and Schumm completely over- can vary from one river to another. The last cited paper shows, for look findings published in the professional paper series, to which, the rivers considered, that restoration of the channels to plateau- again, Schumm has himself contributed and in which 452-B is in top level indicates originally straight traces, confuting in these in- considerable part concerned precisely with the subsurface explora- stances the hypothesis of inheritance. tion of the alluvium/bedrock interface and the identification of My field investigations of the alluvium/bedrock interface in in- former meandering channels, complete with pool-and-riffle se- cised meandering valleys and of the long profiles of channels cut in quences, developed in bedrock. bedrock have been ongoing since 1951. Results up to 1961 are For the record, a reanalysis of the field logs of subsurface explo- brought together in two U.S. Geological Survey Professional Papers ration will serve to re-emphasize the homology between the former (Dury, 1964a, 1964b; see also Dury, 1965). Deliberate subsurface channels associated with incised meanders and present-day chan- exploration of the alluvium/bedrock interface relates to 18 valley nels cut in alluvium (Dury, in prep). Specifically, the former (bed- reaches in England and 5 in the United States. It has involved the rock) channels display width/wavelength ratios that do not differ sinking of 800 or more boreholes and the logging of at least 40 significantly from the corresponding ratios of alluvial streams. lines of seismic sounding on some 120 cross sections. In all cases, the large former channels display pool-and-riffle sequences — not REFERENCES CITED merely possible analogues, but actual homologues — of the sequences that typify meandering channels of the present day. Chorley, R. J., ed., 1969, Water, Earth and man: London, Methuen, 588 p. Investigations since 1961 have been directed in part to the lon- Dury, G. H., 1954, Contribution to a general theory of meandering valleys: gitudinal bed profiles of streams that possess valley meanders but Am. Jour. Sci., v. 252, p. 193-224. not stream meanders. Rock-cut pools occur on the Colo River in 1964a, Principles of underfit streams: U.S. Geol. Survey Prof. Paper New South Wales, Australia, the study of which led to the naming 452-A, 67 p. of the Osage-type underfit (Dury, 1966). The River Severn near 1964b, Subsurface exploration and chronology of underfit streams: Shrewsbury, England, is also an Osage-type underfit (Dury and U.S. Geol. Survey Prof. Paper 452-B, 56 p. 1965, Theoretical implications of underfit streams: U.S. Geol. Survey others, 1972). On at least one reach, pools on the present channel Prof. Paper 452- C, 43 p. are cut in bed rock. A length of some 86 km of the lower Hawkes- 1966, Incised valley meanders on the lower Colo River, New South bury River, New South Wales, Australia, contains at least 40 Wales: Australian Geographer, v. 10, p. 17—25. rock-cut pools, with highly irregular floors and depths as great as 1969, Tidal stream action and valley meanders: Australian Geographi- 30 m below the top of the bank level (Dury, 1970). It is clear that cal Studies, v. 7, p. 49-56.

Geological Society of America Bulletin, v. 87, p. 319-320, February 1976, Doc. no. 60218.

319

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/2/320/3429427/i0016-7606-87-2-320.pdf by guest on 30 September 2021 320 SHEPHERD AND SCHÜMM

Dury, G. H., 1970, A re-survey of part of the Hawkesbury River, New geomorphology: San Francisco, W. H. Freeman, 522 p. South Wales, after one hundred years: Australian Geog. Studies, v. 8, Shepherd, R. G., and Schümm, S. A., 1974, Experimental study of river in- p. 121-132. cision: Geol. Soc. America Bull., v. 85, p. 257-268. Dury, G. H., Sinker, C. A., and Pannett, D. J., 1972, Climatic change and arrested meander development on the River Severn: Area, v. 4, p. 81-85. MANUSCRIPT RECEIVED BY THE SOCIETY APRIL 14, 1975 Leopold, L. B., Wolman, M. G., and Miller, J. P., 1964, Fluvial processes in MANUSCRIPT ACCEPTED JUNE 24, 1975

R. G. SHEPHERD Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 S. A. SCHÜMM Department of Earth Resources, Colorado State University, Fort Collins, Colorado 80S23

We thank Dury for his elaboration of our introductory state- point we tried to emphasize in our introductory paragraphs ments, which were purposefully most general, and for his concur- (Shepherd and Schumm, 1974, p. 257). rence with our brief conclusions concerning meander inheritance. Of course, knowledge of depths to the bedrock-alluvium interface REFERENCES CITED in an alluvial valley, when such depths are measured at discrete and widely-spaced points (for example, see Dury, 1964), does not con- Dury, G. H., 1964, Subsurface exploration and chronology of underfit stitute knowledge of the morphologic details of that interface. Con- streams: U.S. Geol. Survey Prof. Paper 452-B, 56 p. sequently, it is possible that Dury's extensive and excellent work on Shepherd, R. G., and Schumm, S. A., 1974, Experimental study of river in- valley meanders has less relevance to our study than he supposes. cision: Geol. Soc. America Bull., v. 85, p. 257-268. Nevertheless, we are pleased that he did not disagree with any of our research results. Dury would undoubtedly agree that more in-

formation concerning the processes that mold the configurations of MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 2, 1975 the floors of bed-rock channels would be beneficial; this was the MANUSCRIPT ACCEPTED JULY 24, 1975

Printed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/2/320/3429427/i0016-7606-87-2-320.pdf by guest on 30 September 2021