Tamarisk and Fluvial Geomorphic Form in Dinosaur National Monument, Colorado and Utah- Effects of Flow Regulation and Implications for Resource Management
Masters Degree Research
Proposal
Greg Larson
The research proposed will investigate the relationship between tamarisk (Tamarix ramosissima) and fluvial geomorphic form in the dam-regulated Green River and the relatively unregulated Yampa River in Dinosaur National Monument. The study segments of both streams are in northwestern Colorado. The effects of dam-caused flow regulation on tamarisk’s distribution along these two comparably-sized streams will be examined at the segment, reach, and depositional environment scales. The findings of this research will be used to develop a map of Lodore Canyon that identifies those areas where tamarisk removal, in conjunction with planned re-operation of Flaming Gorge Dam, is most likely to be successful and unsuccessful. The results of this project will contribute to our understanding of the natural hydraulic controls on tamarisk’s distribution and help guide tamarisk eradication efforts elsewhere.
Since 1954, the Green River channel through Dinosaur National Monument (Figure 1) has narrowed by an average of about 22% (Grams and Schmidt 2002). Much of this narrowing has been concomitant with invasion by tamarisk (Tamarix ramosissima), a non-native riparian shrub (Graf 1978, Grams and Schmidt 2002). The period during which narrowing occurred also coincided with reductions in peak flows on the Green River resulting from both large scale climate change (1930’s) and the closure of Flaming Gorge Dam (post-1962), located 70 km upstream.
Several studies have examined the role of tamarisk in observed channel narrowing of unconfined alluvial reaches on the Green River downstream from Dinosaur National Monument, and results have been conflicting. Graf (1978) attributed observed narrowing to tamarisk, while others have found that the role of tamarisk is unclear (Everitt 1979, Allred and Schmidt 1999). Much of the narrowing of the Green River through Dinosaur National Monument is due to the formation of inset depositional features as the channel has adjusted to a sediment flux and flow regime altered by Flaming Gorge Dam (Grams and Schmidt 2002). Tamarisk’s post- dam success in Lodore Canyon is likely a result both of colonization of these newly-formed features, and reduced mortality from flooding.
The physical controls of tamarisk’s streamward extent are poorly understood, despite being of both scientific and management importance. Streamward extent is defined as the location of these plants on the banks of a river. Plants that occur at lower elevation and which contribute to channel narrowing occur streamward of others. Defining these controls would help predict possible trajectories of change in channel form and riparian community composition, both as tamarisk invades new areas, and where flow regimes are altered by dams or climate change. Determining these controls would also guide future tamarisk removal projects, allowing land managers to focus on those areas unlikely to be re-colonized under various flow regimes. Design of channel maintaining flows would also benefit from knowledge of what controls tamarisk’s streamward extent.
The research proposed is based on the hypothesis that tamarisk’s streamward extent is controlled primarily through hydraulic forces and the resulting sediment transport. This assumption seems to be supported by the low density of tamarisk in steep canyons with high ratios of peak to base flow (J.C. Schmidt, personal communication), and the plant’s relatively high density in sheltered and low energy environments (Irvine and West 1979). It is possible that changes in flood regime to control the streamward extent of tamarisk could also affect its upslope extent by changing soil moisture conditions or other processes (Graf 1982, Potter et al 1983, Sher et al 2000). Although this study may contribute to our understanding of these processes, they will not be the focus of our work.
Eradication of tamarisk is of practical concern to land managers along the Green River and elsewhere. While some control methods such as herbicides can be effective (Brock 1994), they are less desirable to managers of ecologically sensitive quasi-wilderness areas like Dinosaur National Monument, where this study’s research is proposed. Because tamarisk’s establishment in some areas has been concurrent with decreases in peak flows and the resulting decrease in stream power and fluvial reworking of banks and bars, several authors (Graf 1978, Birkeland 1996, Sher et al 2000, Irvine and West 1979, Stromberg 1997) have suggested that flooding could be a useful means of either eradicating tamarisk or preventing its establishment. The concept of using flooding as a means of preventing vegetation encroachment is supported by many studies that relate vegetation community structure to specific fluvial landforms (Hupp 1983, Hupp and Osterkamp 1996, Bendix and Hupp 2000) and to specific areas of high or low stream power (Bendix 1999). However, flooding may not be effective in removing established tamarisk stands, and may need to be used in conjunction with mechanical shrub removal.
Managers of areas already invaded by tamarisk or prone to invasion would benefit from more information concerning the role this invasive plant plays in changing the geomorphic process of river corridors, which could be used to prioritize locations for removal efforts. Although tamarisk seems to play a role in channel narrowing, bank and bar stabilization, and increased sedimentation, the mechanisms of these processes, and their relationship to geomorphic organization of river corridors are poorly understood.
The proposed study has four main objectives:
1) Relate the distribution of tamarisk in Yampa Canyon and Lodore Canyon to their large-scale geomorphic organization along their lengths- including river gradient, dominant bank substrate, width of the alluvial valley, hydraulic geometry, frequency of island and bars, degree of debris fan influence, and dominant hydraulic control.
2) Relate the distribution of tamarisk in Yampa Canyon and Lodore Canyon to specific geomorphic landforms and depositional environments (Figure 3).
3) Estimate the characteristic depth and duration of inundation for each major geomorphic surface in several detailed study reaches.
4) Synthesize our results, relating the distribution of tamarisk to geomorphic form at the segment, reach, and depositional environment scales, to make management recommendations predicting where tamarisk eradication efforts are most likely to achieve long-term success under proposed re-operation of Flaming Gorge Dam.