Chapter 5 Large-Scale Erosional and Depositional Features of the Channeled Scabland VICTOR R. BAKER Department of Geological Sciences The University of Texas at Austin Austin, Texas 78712 ABSTRACT PRECEDING PAGE BLANK NOT FILMED The Channeled Scabland is a great anastomos- The depositional mesoforms for major channel ing complex of highly overfit steam channels flows were giant current ripples varying from 18 eroded into the basalt bedrock and overlying sedi- to 130m in chord length and from 0.5 to 7m in ments of the Columbia Plateau. Both the ero- height and composed predominantly of gravel. sional and depositional bed forms in these chan- The bed forms may be empirically related to nels can be described according to a simple depth-slope, mean velocity, and stream power. hierarchical classification. The catastrophic flood Correlation coefficients for the relationships are flows produced macroforms (scale controlled by all greater than 0.9. Nevertheless, the prediction channel width) through the erosion of rock and of these hydraulic parameters from ripple dimen- sediment and by deposition (bars). Mesoforms sions applies only to the narrow range of flow (scale controlled by channel depth) are also ero- conditions which characterized the Missoula sional and depositional. Microforms (scale con- Flood through the reaches containing the bed trolled by the inner part of the turbulent boundary forms. layer) are not discussed. Large-scale bedrock macroforms are crudely developed, especially in the Cheney-Palouse scab- land tract. The streamlined residual forms in REGIONAL CHANNEL PATTERNS loess are the most striking macroforms. Their characteristic elongation of 3 times their maxi- J. H. Mackin has been quoted as saying, "to mum width results from the balancing of skin understand the scabland, one must throw away resistance and pressure drag factors to create an textbook treatments of river work" (Bretz and equilibrium landform. The scabland bars are the others, 1956, p. 960). Certainly a failing of characteristic depositional macroforms. Pendant Bretz' critics in the Spokane Flood debate was bars, the most common type, accumulated their insistence that the Channeled Scabland con- wherever large flow separations were generated form to "established" geomorphic processes. The by various flow obstructions or diversions. scale of the problem was key, and a completely The bedrock mesoforms include longitudinal new frame of reference was required. Bretz grooves, potholes, rock basins, inner channels, (1932a, p. 28) provided the required viewpoint: and cataracts. A sequence of bed form develop- "Channeled Scabland is river bottom topography ment probably operated for bedrock erosion in magnified to the proportion of river-valley topog- the Channeled Scabland. With time the forms raphy." changed from (1) longitudinal grooves, to (2) The Spokane Flood debate might have been rock basins and potholes, (3) inner channels with resolved more easily if the participants could have recessional cataracts at their heads, and finally viewed modern orbital photography of the region to deeply incised inner channels. (Fig. 5.1). At a glance one can appreciate Bretz' 81 Preceding page blank observation. Clearly the Channeled Scabland is a or bedrock streams. The scabland anastomosis is plexus of channels rather than a network of deeply cut into rock. Anastomosis occurs in the valleys. Channeled Scabland because preflood valleys did not have the capacity to convey the Missoula Channel Anastomosis Flood discharges without spilling over preflood divides into adjacent valleys. This crossing of The general pattern of the Channeled Scabland divides produces the effect of channels dividing is large channels eroded in loess and underlying and rejoining. Before the era of aerial photo- basalt. The channels form locally anastomosing graphs and adequate topographic maps, Bretz complexes with individual channels that have rel- (1928b) used field surveys to show over 100 atively low sinuosity. The term "anastomosis" channnel ways in the scablands, SO of which bi- should not be confused with "braiding" (as was furcated in a downstream direction. Modem data done in the A. G. I. Glossary of Geology). sources now show that anastomosis occurs on a "Braiding" refers to branching and rejoining variety of scales. The regional scale (Fig. 5.2) is around alluvial islands or bars. Braided streams controlled by preflood topography. Individual are part of a continuous series of fluvial forms channels may also be controlled by relatively that develop in quasi-equilibrium with external straight geologic structures, such as High Hill controls on the river systems. "Anastomosis" has anticline in the Channeled Scabland. Small-scale no genetic connotation. It refers to channel mor- anastomosis includes the minor divide crossings phology whether in alluvial streams ("braided") used for reconstruction of the flood high-water surface. STREAM OVERFITNESS Misfit streams are streams that are either too small or too large for the valleys in which they flow (Dury, 1958, 1964). The underfit variety is relatively common, and such streams often show smaller channel widths and meander wave- lengths than the winding valleys in which they flow. The disparity between river and valley size is explained by a reduction in stream discharge, either by capture (Davis, 1913) or by climatic change (Dury, 1965). Although there is consid- erable debate over the validity of the climatic ex- planation of underfit streams, the concept does seem to apply to alluvial valleys (Baker and Penteado-Orellana, 1977). An overfit stream is too large for the valley in which it flows. Dury (1964) considered overfit streams in the context of sudden increases in dis- charge with rapid channel enlargement. Although such a condition would not persist long in an alluvial valley, the erosion of bedrock probably provides an opportunity to preserve overfit stream relationships. Figure 5.1. Anastomosing channel pattern in the Tel- ford-Crab Creek scabland complex. This LANDS AT The Crab Creek area, near the town of Wilson image depicts a scene 70 x 150 km (LANDSAT E-1039- Creek in the Channeled Scabland, provides an ex- 18143-5, 31 August 1972). cellent example of overfit relationships introduced 82 " j j j j j A.of~ j lUis of $JnCIine j j fOIIl j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j j by catastrophic scabland flooding. (See Bretz and ervation throughout the scablands. The larger others, 1956, their Plate 8; Baker, 1973a, his features are scaled to channel width and consist Fig. 22.) Prior to the catastrophic flooding, the either of depositional "bars" or various kinds of topography near Wilson Creek was probably very erosional residuals in rock or sediment. The similar to that of the modern Palouse Hills region smaller features are. scaled to the phenomenal near Pullman, Washington, with the interstream scabland flow depths. divides thickly mantled by the Palouse loess. The Jackson (1975) has developed a hierarchical major streams had been superimposed onto the classification of bed forms generated by fluid shear basalt from this loessal cover. The streams flowed and composed of cohesionless granular material. in relatively narrow valleys. The well-formed val- The classification relates to bed form size and to ley meanders had a wavelength of 2000m. Using the time span of existence for various bed con- empirical relationships that characterize most figurations. The bed form groups each relate to rivers, the normal bankfull discharge of Crab different formative processes (Jackson, 1977). Creek was approximately 850 ms/sec. Macroforms for rivers include point bars, scroll The last major scabland flood completely filled bars, alternate bars, and pool-and-riflle se- the valley of Crab Creek and adjacent streams. quences. These bedforms do not relate to local Water spilled over many of the divides between flow conditions. They rather respond to long- the stream valleys. The Missoula flood flows in term hydrologic and geomorphic factors. Meso- this area where approximately 2,800,000 mVsec forms include large-scale ripples (dunes), anti- (Baker, 1973a, his Plate 1). Bretz (1928b) dunes, and large-scale lineation. The spacing of noted that the flooding could not tolerate the mesoforms depends on the outer zone of the tur- leisurely preftood curves of the incised stream. bulent boundary layer as the flow varies through Slip-off slopes were vigorously attacked produc- a dynamic event such as a flood. In rivers the ing what he called "trenched spur buttes". Huge boundary layer control is approximated by flow streamlined bars were deposited downstream depth. Microforms include current lineation and from the former valley bends. Many of these have small-scale ripples. Microforms respond to flow giant current ripples on their upper surfaces (Fig. structure in the inner part of the turbulent bound- 5.3). The preflood topography was reduced to ary layer, and their lifetime is much shorter than mere bottom roughness elements by flooding 3 the periodicity of dynamic events. to 4 orders of magnitude greater than any flooding Jackson's classification is especially interesting these streams had ever experienced. In the upper because of its genetic implications. Heretofore parts of the Wilson Creek drainage, the scabland most hydrodynamic bed form studies have fo- erosion presents a striking contrast to the adjacent cused on the unstable motion of water over a loess-mantled terrain (Fig. 5.4). rough boundary generating variable shear stress. Flow properties are perturbed either into longi- tudinal vortices that produce flow separation or into transverse roller vortices that produce alter- A HIERARCHY OF SCABLAND BED FORMS nating separation and reattachment of stream- lines along the boundary (Allen, 1971a, 1971b). A fluvial bed form is defined (Am. Soc. Civil Until recently there has been little correspondence Engrs., 1966) as follows: "any deviation from a between experiments and theory. Jackson (1976, plane bed that is readily detectable by eye or 1977) advocates a resolution to this dilemma higher than the largest sediment size present in through the concept of flow structures.