Terraces and alluvial fans of the Madison Valley, SW MontanaWilliam Locke
Montana State University
Continent: North America
Country: United States of America
UTM coordinates and datum: 12T VR 5323 2122
Climate Setting: Semi-Arid
Tectonic setting: Continental Rift
One of the classic visual examples in all of geomorphology is the well-labeled Ennis, MT, topographic map of the Cedar Creek Alluvial Fan (Fig. 1) - a spectacular apron of stream-deposited sediment. This topographic map is included in most high school and introductory college earth science textbooks and laboratory manuals. The Cedar Creek fan, however, is far more complex than it first appears and is nestled in an environment that holds a suite of classic examples of the value of fluvial (river-formed) materials and landforms in the interpretation of the evolution of landscapes across time.
Is the Cedar Creek fan as classic as it appears? In detail - no. Better models for a purely depositional alluvial fan are those of Shell and Tolman creeks, 5-10 km to the south of Cedar Creek (Fig. 2). Estimates of ages of various surfaces of the Cedar Creek fan by Ritter and others (1993) show that it is composed of a post-glacial (last 14,000 years) channel deeply inset into a lastglacial southern segment that is in turn cut into (near the head) and built across (near the toe) an older segment assigned to the penultimate glaciation (~140,000 years old) (Fig. 3). Clearly, the Cedar Creek fan is alternately being built and eroded away by the stream that formed it, with erosion dominating the recent geological history (Fig. 4).
Five km to the north of Cedar Creek is Jack Creek (Fig. 5). Where Cedar Creek hosts its massive, complex fan, Jack Creek displays a flight of terraces: abandoned river floodplains subparallel to the modern stream but rising hundreds of meters above it (Fig. 6). Flights of terraces are evidence of long-term downcutting alternating with periods of stability or deposition (Bearzi, 1987). Why does Cedar Creek have a fan and Jack Creek have terraces, given that they share adjacent drainage basins with comparable geology and tectonic history?
Any stream will, given sufficient time, develop a local slope that is adequate to carry the load of gravel, sand, silt, and clay provided to it by erosion of the slopes in its drainage basin with the flow of water provided by springs, rainstorms, and snowmelt. The upper reaches of streams are steep, to carry the large, coarse load with the small amount of runoff available in the fingertip tributaries. Conversely, the lower reaches of river systems have almost imperceptible slopes, but still enough to carry their relatively fine sediment load with the immense amounts of water collected from their entire basin. The drainage basin of Jack Creek is several times larger than that of Cedar Creek, providing more water to carry the sediment load and requiring a lower slope to accomplish that task. But both streams are tributaries to the Madison River at about the same elevation. Thus Jack Creek exits the Madison Range-front at a much lower elevation (about 5440') than does Cedar Creek (~6080') and has not needed to build an alluvial fan "ramp" to carry itself and its load from the mountains to the major river that will, in the end, carry that load to the Gulf of Mexico.
A final question from this landscape is why Jack Creek has so many terraces (about nine) with slopes that vary by up to 50% (0.019 to 0.030), whereas Cedar Creek has only three or four surfaces (again with slopes differing by about 50%). Each surface represents an approach to the equilibrium between load and discharge of water: varying slopes thus represent, as a first approximation, variation in either the amount or size of the sediment load or the amount of water available to transport it. Such variations are usually explained (at least in the mountainous regions of the world!) by episodes of glaciation that are caused in part by variation in available water and that result in a pulse of glacially-derived coarse sediment. Both Jack Creek and Cedar Creek were recently glaciated, with that of Cedar Creek much more extensive (almost to the mouth of the canyon and apex of the alluvial fan).
The terraces of Jack Creek (Fig. 6) show former river gradients that approach either 0.02 (modern, interglacial conditions) or 0.03 (former, possibly glacial conditions). They MIGHT represent a sequence of Modern (10) and Holocene (9) interglacial terraces, lastglacial (8, 7, 6), penultimate interglacial or interstadial (5, 4), penultimate glacial (3, 2) and preglacial (1). However, numerical age estimates would be required in order to test that hypothesis.
So - why does Jack Creek show such a range of behaviors, Cedar Creek only three surfaces, and Shell and Talman creeks only one modern surface? Partly, it is the more advanced evolutionary state of the larger basins, with more water, thus more energy, thus less time required to equilibrate. However, it also may relate to base level change. Jack and Cedar creeks drain directly to the Madison River; as the Madison rises or falls in its valley with glacial deposition or interglacial erosion, or with faulting across its valley floor, the tributaries must adjust (which may also explain some of the terraces of Jack Creek). In contrast, Shell and Tolman creeks have the Cameron Bench (Fig. 7) - a terrace of the Madison River inferred as of penultimate glacial age - as local base level. Changes in the elevation of the Madison River don't affect those tributaries, thus they will continue to deposit until they are near equilibrium, then incise as the mountains are washed to the sea.
The terraces and alluvial fans of the Madison Valley record not only the formation of alluvial fans as a result of the loss of power when a steep mountain stream runs onto a flat basin floor, but a history of climate change and of the effects of basin size in controlling the evolutionary state of alluvial fans. Why should we care? The alluvial piedmont north of Bozeman, Montana (about 60 km NE of the Madison Valley) is rapidly urbanizing (Fig. 8); it would be well to understand the workings of the streams and their alluvial fans before covering them with houses!
- Bearzi, James (1987), Soil development, morphometry, and scarp morphology of fluvial terraces at Jack Creek, SW Montana. Unpubl. MS thesis, Montana State University, Department of Earth Sciences, 131 pp.
- J.B. Ritter, J.R. Miller, Y. Enzel, S.D. Howes, G. Nadon, M.D. Grubb, K.A. Hoover, T. Olsen, S.L. Reneau, D. Sack, C.L. Summa, I. Taylor, K.C.N. Touysinhthiphonexay, E.G. Yodis, N.P. Schneider, D.F. Ritter and S.G. Wells (1993), Quaternary evolution of Cedar Creek Alluvial Fan, Montana, Geomorphology, 8 (1993) 287-304 287