Delayed Effects of Flood Control on a Flood‐dependent Riparian Forest

Abstract

The downstream effects of dams on riparian forests are strongly mediated by the character and magnitude of adjustment of the fluvial-geomorphic system. To examine the effects of flow regulation on sand-bed streams in eastern Colorado, we studied the riparian forest on three river segments, the dam-regulated South Fork Republican River downstream of Bonny Dam, the unregulated South Fork Republican River upstream of Bonny Dam, and the unregulated Arikaree River. Although Bonny Dam significantly reduced peak and mean discharge downstream since 1951, there was little difference in forest structure between the regulated and unregulated segments. On all river segments, the riparian forest was dominated by the native pioneer tree, Populus deltoides, which became established during a period of channel narrowing beginning after the 1935 flood of record and ending by 1965. The nonnative Elaeagnus angustifolia was present on all river segments, with recruitment ongoing. The lack of contrast in forest structure between regulated and unregulated reaches resulted primarily from the fact that no large floods occurred on any of the study segments since dam construction. Most of the riparian forest in the study area was located on the broad narrowing terrace, which was rarely inundated on the unregulated segments, resulting in little contrast with the regulated segment. A minor dam effect occurred on the small modern floodplain, which was actively disturbed on the unregulated segments, but not on the regulated segments. Although Bonny Dam had the potential to significantly influence downstream riparian ecosystems, this influence had not been expressed, and may never be if a large flood does not occur within the lifetime of the dam. Minor dam effects to riparian systems can be expected downstream of large dams in some settings, including the present example in which there was insufficient time for the dam effects to by fully expressed.

Cottonwood understory zonation and its relation to floodplain stratigraphy

“…The larger size of Russian olive individuals in the open could have been due either to earlier establishment in these areas or to faster growth there. However, Katz et al (2005) found no evidence that Russian olive had established earlier in open habitats than in the cottonwood understory in our study area. Thus, the larger size of Russian olive individuals in the open was likely due to faster growth rates where light levels were 1.5 times higher than under the cottonwood canopy.…”

“…Few other tree or shrub species are present at these sites, and these are only rarely found. For example, Katz et al (2005) found that the next most common tree or shrub at our sites, Juniperus virginiana L., represented only 0.6% of the individual trees or shrubs present in the study area. At each site, Russian olive stands extended at least 100 m away from the river and at least 400 m along the river's length.…”

“…monilifera (Aiton) Eckenwalder] and peachleaf willow (Salix amygdaloides Anderss.) on the flood-widened channel bed (Katz et al 2005). These are patchily distributed across the floodplain, with patches ranging from single trees to several hectares in size interspersed with areas of open meadow.…”

Six decades of changes in the riparian corridor of a Mediterranean river: a synthetic analysis based on historical data sources

“…According to Katz et al (2005), in the North American Great Plains, dams typically have reduced both downstream peak discharge and sediment load, resulting in diminished stream power downstream and a narrower active river channel. When dams retain sediment and reduce discharge, in the long term, there is a reduction in the channel migration rate because those flows able to produce erosion and point bar deposition are no longer released (Friedman et al, 1998;Johnson, 1998).…”

Riparian vegetation patterns according to hydrogeomorphological factors at different spatial and temporal scales in Mediterranean rivers.

“…In the North American Great Plains (Katz et al, 2005), dams typically have reduced both downstream peak discharge and sediment load, resulting in diminished stream power downstream and a narrower active river channel. When dams retain sediment and reduce discharge, in the long term, there is a reduction in the channel migration rate because those flows able to produce erosion and point bar deposition are no longer released (Friedman et al, 1998;Johnson, 1998).…”