Complex interactions between channel morphology and invasive riparian plants in Canyon de Chelly, Arizona

Ephemeral channels in the semi-arid Colorado Plateau region are subject to a cyclic process of cutting and filling resulting from variation in precipitation patterns (eg. Leopold, 1976), variation in land cover (eg. Antevs, 1952), and from inherent thresholds of instability in the system (eg. Schumm, 1979). Streams often alternate between wide, braided channels with shallow water tables at one extreme, and narrow, meandering, incised channels with a distinct rectangular cross section called arroyos on the other extreme. This cycle can occur on time scales ranging from decade to centuries. These changes can disrupt human use of the floodplain, for example in Canyon de Chelly National Monument, in northeastern Arizona, where channel incision has lowered the water table to the extent that the resident Navajo community is no longer able to practice traditional farming. Even small climatic changes can alter the ecological character of southwestern watersheds (Turner, 1974), and both precipitation and vegetation changes have the potential to affect channel form (Kondolf et al., 2002). For example, wetter conditions may enable vegetation to grow more densely across the landscape, which will in turn prevent soil from being washed into the stream, reducing the sediment load of the stream thereby increasing the ability of the stream to erode its bed or banks. Even without changes in precipitation patterns, the semi-arid climate may lead to inherent instability in channel form. Schumm and others have suggested that the high sediment production and infrequent flows typical of rivers on the Colorado Plateau lead to sediment accumulation in channels and steepening of the bed slope until an intrinsic threshold for bed incision is crossed, resulting in episodic sediment transport and erosion (Patton and Schumm, 1981; Schumm and Hadley, 1957). Anthropogenic (i.e., human caused) influences, such as farming, road building, and dam construction, add additional complexity (Phippen and Wohl, 2003; Williams and Wolman, 1984) (Figure 1).

Our understanding of this cycle of erosion has been complicated by the widespread invasion of exotic woody riparian plant species, in particular tamarisk (Tamarix spp.) and Russian olive (Elaeagnus angustifolia). Although morphological changes are not uniform across the Colorado Plateau, Canyon de Chelly has experienced channel narrowing and incision as well as invasion by both tamarisk and Russian olive over the past 40 years, providing a setting in which to analyze the interaction between these two types of change (Figures 2 & 3). Repeat ground photos demonstrate the stark changes in the canyon (Figure 4). We initially hypothesized that exotic vegetation establishment on the floodplain and channel surface corralled flow into a smaller channel and increased erosive forces on the stream bed, causing down-cutting. We analyzed 6 air photo sets, from the years 1935, 1964, 1975, 1981, 1989, and 2004, surveyed the longitudinal profile of the incised channel, as well as the slopes of all major terraces (abandoned floodplains), and cored cottonwood, Russian olive, and tamarisk trees to determine when they germinated.

The air photo analysis showed that the streams were wide (100 m on average) and braided throughout the study area in 1935. Narrowing occurred earliest in the uppermost reaches of the study site, and progressed farther downstream in each subsequent photo set. Narrowing in the upper reaches occurred before widespread vegetation was visible in the air photos. Vegetation establishment patterns were much more complex: cottonwood established in bands along the stream banks prior to 1964, and in wide stands near the transition from recently narrowed reaches to braided reaches. From 1981 to 2004 tamarisk occupied the former channel bed in a downstream-progressing wave, simultaneous with channel narrowing (Figure 5). These reaches are the most deeply incised, up to 5 m below the 1935 stream bed, and are only 7 m wide on average. Between 1989 and 2004, Russian olive established along the previously sparsely vegetated upper reaches.

These observations suggest that although tamarisk establishment coincided with arroyo formation, it did not force or cause this narrowing. Rather it interacted with and exacerbated an already occurring sequence of changes. Other factors besides invasive vegetation appear to be the initial causes of channel change. For example, reduced herd sizes on the Navajo Reservation beginning in the 1940s may have decreased soil erosion rates, reducing sediment supply to the channels and triggering incision. Or increased winter precipitation and resultant spring runoff floods may have increased erosion capacity in the stream. Nonetheless, the invasive vegetation appears to have strengthened the banks, causing the incised channel to maintain steeper walls and increasing erosive forces on the stream bed.

• Antevs, E.V., 1952. Arroyo-cutting and filling. Journal of Geology, 60(4): 375-385.
• Kondolf, G.M., Piegay, H. and Landon, N., 2002. Channel response to increased and decreased bedload supply from land use change: contrasts between two catchments. Geomorphology, 45(1-2): 35-51.
• Leopold, L., 1976. Reversal of erosion cycle and climatic change. Quaternary Research, 6(4): 557-562.
• Patton, P. and Schumm, S.A., 1981. Ephemeral-stream processes: Implications for studies of quaternary valley fills. Quaternary Research, 15(1): 24-43.
• Phippen, S.J. and Wohl, E., 2003. An assessment of land use and other factors affecting sediment loads in the Rio Puerco watershed, New Mexico. Geomorphology, 52(3-4): 269-287.
• Schumm, S.A., 1979. Geomorphic thresholds: the concept and its applications. Transactions of the Institute of British Geographers, 4(4): 485-515.
• Schumm, S.A. and Hadley, R.F., 1957. Arroyos and the semiarid cycle of erosion. American Journal of Science, 255: 161-174.
• Turner, R.M., 1974. Quantitative and Historical Evidence of Vegetation Changes Along the Upper Gila River, Arizona. U.S. Geological Survey Professional Paper 655-H. U.S. Government Printing Office, Washington D.C., pp. 20.
• Williams, G.P. and Wolman, M.G., 1984. Downstream Effects of Dams on Alluvial Rivers. U.S. Geological Survey Professional Paper 1286. U.S. Government Printing Office, Washington D.C., pp. 83.