Abstract

Many field indicators have been used to approximate the magnitude and frequency of flows in a variety of
streams and rivers, yet due to a scarcity of long-term flow records in tropical mountain streams, little to
no work has been done to establish such relationships between field features and the flow regime in
these environments. Furthermore, the transition between the active channel of a river and the adjacent
flood zone (i.e. bankfull) is an important geomorphologic and ecological boundary, but is rarely identifiable
in steep mountain channels that lack alluvial flood plains. This study (a) quantifies relationships
between field indicators and flow frequency in alluvial and steepland channels in a tropical mountain
stream network and (b) identifies a reference active channel boundary in these channels, based on statistically
defined combinations of riparian features, that corresponds to the same flow frequency of the
bankfull stage and the effective discharge in adjacent alluvial channels. The relative elevation of transitions
in riparian vegetation, soil, and substrate characteristics were first surveyed at nine stream gages in
and around the Luquillo Experimental Forest in Northeastern Puerto Rico. The corresponding discharge,
flow frequency, and recurrence intervals associated with these features was then determined from longterm
15-min discharge records and a partial duration series analysis. Survey data indicate that mosses
and short grasses dominate at a stage often inundated by sub-effective flows. Herbaceous vegetation is
associated with intermediate discharges that correspond to the threshold for sediment mobilization.
Near-channel woody shrubs and trees establish at elevations along the channel margin inundated by a
less frequent discharge that is coincident with the effective discharge of bed load sediment transport.
Our data demonstrate that in alluvial channels in the study, both the bankfull stage (as marked by a flood
plain) and the channel-forming (effective) discharge are associated with the presence of fine-grained substrate
and soil, and tall, mature woody vegetation. In montane reaches that lack a flood plain, a boundary
that is characterized by the incipient presence of soil, woody shrubs, and trees corresponds to the same
flow frequency as the bankfull discharge of nearby alluvial channels. The reference discharge based on
these riparian features in steepland sites has an average exceedance probability between 0.09% and
0.30%, and a recurrence interval between 40 and 90 days. We conclude that flows with similar frequencies
influence the establishment of riparian vegetation, soil development, and substrate characteristics
along channel margins in similar ways. Thus, these riparian features can be used as an indicator of
hydrogeomorphic site conditions to identify active channel boundaries that occur at a constant flow frequency
throughout the study stream network.

Citation

Pike A.S., F.N. Scatena (2010): Riparian indicators of flow frequency in a tropical montane stream network. Journal of Hydrology .