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

Hydrograph Characteristics Relevant to the
Establishment and Growth of Western
Riparian Vegetation

M.L. scotti, M.A. Wondzel12, G.T. AUblel

ABSTRACT

In arid and semi-arid regions of western North
Americar natural regeneration of woody riparian
vegetatlon is dependent on adequate moisture.
Because water is often limiting, the timing and
pattern of water delivery are crucial to the long-
term maintenance of riparian vegetation. An
examination of how a hydrogra~h satisfies the
ecological requirements of riparlan species is an
important step in assessing potential impacts of flow
alterations.

Riparian species such as cottonwood and willow
disperse seeds over a two- to six-week time period.
Seed dispersal for cottonwood and willow typically
coincides with peak flows resulting from snowmelt
runoff or spring thunderstorms. Dispersed seeds lose
germinabillty rapidly: thus, seeds must encounter
suitable germination sites soon after release. In
addition to timing of ~eak flows, the magnitude of
the peak flow is critlcal in preparing seed beds
suitable for germination. Germination and
establishment typically take place on freshly
deposited alluvium in channel positions low enough to
provide adequate moisture but high enough to escape
scour from subsequent floods and ice.
Characteristically,. establishment occurs
following medium to large floods. Once seedlings are
established, root growth must keep pace with
declining river. stage and water table. If river
stages decline too rapidly, drou9ht stress produces
substantial seedling mortallty. However,
manipulative experiments indicate that some
cottonwood seedllngs can keep pace with alluvial
water table declines of 8 em/day and by the end of
the first growing season may send a tap root to
depths approachin9 1 m. Provided drawdown does not
greatly exceed ~hlS rate and base flow conditions do
not cause alluvial groundwater to retreat below 1 m,
seedling recruitment is likely to be successful.
Lower peak flows in years immediately following
establishment also contribute to long-term survival
of cottonwood and willow. with increasing size,
cottonwood and; willow become more resistant to
removal by scouring and burial by sediments.

IU.S. Fish and wildlife Service, National Ecology
Research Center, 4512 McMurry Ave., Fort Collins, co
80525-3400, U.S.A.

2U.S. National Park Service, Water Resources
Division, 1201 Oak Ridge Dr., Suite 250, Fort
Collins, CO 80525, U.S.A.

237