Size and lithology of the channel bed material: how channel and watershed properties are related in the Colorado Front Range, USA MSc Thesis Report Foeke Menting January 2014 ii Size and lithology of the channel bed material: how channel and watershed properties are related in the Colorado Front Range, USA MSc Thesis Report Foeke Menting BSc (reg.nr. 891107558120) Wageningen University June 2013 - January 2014 Supervisor: Arnaud Temme 2nd supervisor: Abigail Langston Course code: SGL-80436 iii iv Abstract To get a better insight into the processes controlling the size and lithological distributions of channel bed material in mountainous environments this study is conducted based on a field survey in the Colorado Front Range. The objective is to relate observed size and lithological distributions of the channel bed material to local channel properties and to the position in the landscape. 61 locations have been surveyed along five creeks using the Wolman pebble count. The study shows that the lithological distribution of the bed material in mountainous areas is influenced by lateral inputs of hill slope processes. In absence of lateral inputs the bed material is more a representation of the lithological distribution of the whole watershed. Fining occurs on all investigated scale levels, but not always. This is most likely related to differences in discharge characteristics per creek. No reliable conclusions can be drawn for fining of specific lithologies due to a limited amount of data and varying channel properties. Size sorting of the channel bed material gets disrupted by lateral inputs. Sorting mainly occurs in channel sections without significant lateral inputs. The main conclusion of the study is that in absence of lateral inputs the behaviour of the channel bed material is controlled by basic hydrological relations. Lateral inputs and specific discharge characteristics strongly influence the composition of the channel bed material. Keywords: Front Range, fluvial geomorphology, channel properties, channel bed material, pebble sizes, lithology, fining, sorting v vi Table of Contents 1 Introduction ........................................................................................................................ 1 2 Background ......................................................................................................................... 2 3 Hypotheses ......................................................................................................................... 8 4 Study Area .......................................................................................................................... 9 5 Methods ........................................................................................................................... 12 5.1 Study Site Description ................................................................................................ 12 5.2 Field survey ................................................................................................................ 12 5.3 GIS .............................................................................................................................. 13 5.4 Sedimentary links, tributary junctions and individual lithologies ............................. 14 5.5 Statistics ..................................................................................................................... 16 6 Results .............................................................................................................................. 19 6.1 Field Survey ................................................................................................................ 19 6.2 Data Exploration ........................................................................................................ 19 6.3 Individual Creek Characteristics ................................................................................ 26 6.4 Research Question 1 .................................................................................................. 32 6.5 Research Question 2 .................................................................................................. 34 6.6 Research Question 3 .................................................................................................. 38 7 Discussion ......................................................................................................................... 40 7.1 General Characteristics .............................................................................................. 40 7.2 Lithological Relations ................................................................................................. 42 7.3 Fining ......................................................................................................................... 45 7.4 Sorting ........................................................................................................................ 49 7.5 Limitations ................................................................................................................. 50 7.6 Future Research ......................................................................................................... 53 8 Conclusions ....................................................................................................................... 55 Acknowledgement.................................................................................................................... 57 References ................................................................................................................................ 58 Appendix I – Chi-Squared test results ...................................................................................... 61 Appendix II – 7 Graphs comparing lithological contributions .................................................. 64 Appendix III – RQ2: smallest scale level results ....................................................................... 66 vii viii List of Figures Figure 1-Schematic diagram of the evolution of alluvial channels through time ___________ 2 Figure 2-Schematic diagram of the evolution of bedrock channels through time __________ 2 Figure 3-Schematic depictions of plucking and abrasion _____________________________ 3 Figure 4-Location of the Colorado Front Range ____________________________________ 9 Figure 5-Geology of the watersheds of the creeks and the sampling locations ___________ 11 Figure 6-Schematic depiction of the difference between the total watershed and the local watershed ________________________________________________________________ 14 Figure 7-Longitudinal profiles of the sampled sections of all the creeks under investigation 20 Figure 8-Longitudinal profile of Boulder Creek ____________________________________ 20 Figure 9-Size of watershed for each sampling location _____________________________ 21 Figure 10-Channel cross-sectional area (m2) of all sampling locations _________________ 22 Figure 11-The measured slopes of all sampling locations ___________________________ 23 Figure 12-Watershed size per location per creek versus bankfull width and the channel cross- sectional area _____________________________________________________________ 24 Figure 13-D50's (median grain sizes) in Phi units (log2), of all sampling locations _________ 25 Figure 14-Sedimentary rock counts in South Boulder Creek _________________________ 27 Figure 15-Sedimentary rock counts in Left Hand Creek _____________________________ 28 Figure 16-Geological map of the Left Hand Creek watershed with pie charts ____________ 29 Figure 17-Geological map of the Boulder Creek (NBC, FM, BCM, BCP) watershed with pie charts ____________________________________________________________________ 30 Figure 18-Geological map of the South Boulder Creek watershed with pie charts ________ 31 Figure 19-Watershed size and χ2 values for all sampling locations ____________________ 33 Figure 20-Cumulative size distributions of all the mountain locations and all the plains locations grouped together ___________________________________________________ 34 Figure 21-Trends in D50 for all individual lithological units under investigation ___________ 37 Figure 22-All sorting coefficients of all sampling locations ___________________________ 38 Figure 23-Longitudinal profile of South Boulder Creek _____________________________ 40 Figure 24-Slope map of the Front Range. ________________________________________ 41 Figure 25-Photo of location BCM11 ____________________________________________ 43 Figure 26-Z-values for granitic and metamorphic channel material in Boulder Creek ______ 44 Figure 27-Cumulative size distributions of the mountains points and plains points of Left Hand Creek grouped together ________________________________________________ 46 Figure 28-D50 (Phi) of the mountain and plains points of Left Hand Creek ______________ 47 Figure 29-Image of location LH3 and LH16, illustrating decrease in discharge ___________ 51 Figure 30-Image of location BCM8 and BCP10, illustrating step-pool geometry __________ 52 ix x List of Tables Table 1-Size ranges and size classes ____________________________________________ 13 Table 2-Sequences of numbering of the sampling locations per creek _________________ 19 Table 3-Results of the Mann-Whitney U test on second counts ______________________ 25 Table 4-Mann Whitney U test results expressed in z and P-values per creek on the mountains vs plains scale level _________________________________________________________