9617

• UCREFRP Catalog Number: 9617
• Author: Scott, M. L., M. A. Wondzell and G. T. Auble
• Title: Hydrograph Characteristics Relevant to the Establishment and Growth of Western Riparian Vegetation
• USFW Year: 1993
• USFW - Doc Type: Hydrology Days Publications
• Copyright Material: NO

riparian species like plains cottonwood must keep
pace with a decl ining alluvial water table. In
southern Alberta, Mahoney and Rood (1991) suggest
that survival of riparian poplar seedlings is most
likely in years where drawdown rates do not exceed 4
em/day. Results of the above studies indicate that
cottonwood establishment and early growth are likely
to be successful provided drawdown rates are not
greatly in excess of 4 em/day and base flow
conditlons do not cause alluvial groundwater to
retreat below approximately 1 m.

Longer-term Survival

Bradley and smith (1986) found that recruitment or
establlshment of indivldual cottonwood stands on
river point-bars occurred approximately once in every
5 years. Although peak flows sufficient to support
establishment occurred once in levery 2 years,
coincidence of peak flows with; seed dispersal
occurred less freq!-lently. Lon<;{er"7te~ sur;vival of
cottonwood stands 1S also assoc1ated w1th lmportant
fluvial geomporhic processes. Along the Little
Missouri River in North Dakota, Everit (1968) found
that age and areal distribution of.~ottonwood stands
were closely associated with patterns of channel
movement and flood plain building processes. When a
river sandbar or point-bar attaLned a height of
approximately 1.5 m above low water, cottonwood
seedlings became established on . the bar surface.
Typically, seedlin~s in such locations are subject to
"chronic flood-tralning" (Lindsey et al. 1961) as a
result of sediment deposition during subsequent high
flows. Flood-trained saplings are flattened in a
downstream direction by flood waters and buried by
freshly deposited sediments. New stems arise from
the flood-trained stem but are again flattened and
buried by floods, giving them a stair-step
appearance. With continued deposition and movement
of the channel away from the point-bar, a stem
eventually escapes the zone of flood-training as
fewer floods inundate the aggrading surface. As a
stem continues to increase in girth it tends to
resist the force of a moderate flood, and is likely
to be uprooted and swept away only by a very large
flood.

THE H'iDROGRAPH

Germination and establishment of riparian vegetation
occur in response to the character and pattern of
streamflow w1thin a given year or growing season.
Consequently, an annual hydrograph (as compared to a
mean annual hydrograph) best represents and defines
those critical hydrologic elements that meet the
ecological requirements of riparian plants. A mean
annual hydrograph may obscure the unique behavior of
a stream or river within any given year. The
specific timing, magnitude, and intensity of an
individual peak flow, for example, are muted or lost
when averaged over an extended period of time.
Figure 1, therefore, depicts an annual hydrograph of

240