Geoscience Reference
In-Depth Information
sections showing multiple episodes of degradation and aggradation suggested that
arroyo incision could occur within less than 100 years, but more than 500 years might
be needed for the complete aggradation of valley floors and incised stream chan-
nels within an ephemeral stream system. The lack of synchronism between episodes
of incision in adjacent arroyo systems revealed that local hydrologic factors could
outweigh the influence of regional drought and flood events, except in the case of
extreme floods following sustained droughts in some of the larger ephemeral streams.
An interesting by-product of the research into arroyos has been the realisation that
well-meaning (and expensive) attempts at soil conservation were often either inef-
fective or unnecessary, depending on what stage of development the arroyo system
had reached (Tuan,
1966
; Bull,
1997
).
The role of vegetation change on alluvial deposition in arid areas is in fact more
complex than either Bull (
1997
)orTuckeretal.(
2006
) realised. Antinao and Mc-
Donald (
2013
) studied four localities at different elevations in the Mojave and north-
ern Sonoran deserts. They found that the onset of deposition on alluvial fans began
well before any changes in plant cover upstream, and could occur during several dis-
tinct combinations of vegetation change. They concluded that other factors probably
controlled late Pleistocene and Holocene fan aggradation in this region, including
local storm intensity and changes in the routes taken by water and sediment on hill
slopes.
Vegetation also exerts a powerful control on channel patterns in ephemeral streams,
most noticeably as one of the major causes of anabranching (Tooth and Nanson,
2000
;
Tooth and Nanson,
2004
). In fact, over the past decade, many river channels that were
once regarded as braided have increasingly been identified as anabranching channels,
but the fundamental causes of anabranching have remained enigmatic. A detailed
study of the Marshall River in arid central Australia has shed some useful light on this
question (Tooth and Nanson,
2000
). This work highlights the importance of one small
local tree, the inland tea-tree (
Melaleuca glomerata
), in controlling the anabranching
form of ephemeral stream channels in this arid region. The tea-trees first establish
themselves sporadically within the sandy channels, forming obstacles below which
sandy alluvium accumulates. Small ridges of sand develop within the channel bed,
causing the diversion of flow around them. As the ridges become longer and join with
other mid-channel ridges, a long vegetated ridge develops and divides the channel into
anabranches. This has the effect of decreasing resistance to flow and allows efficient
movement of water and sediment within the anabranch channels. Anabranching can
thus be seen as a means by which ephemeral stream channels can maintain the flow of
water and sediment in situations where the river cannot increase its gradient. Unlike
braided channels, where the mid-channel bars are unstable and ephemeral features,
the vegetated ridges between anabranch channels are stable and relatively permanent
landforms. This study very clearly demonstrates the important effects of vegetation
on fluvial processes in arid areas.
