Evaluation of Gage Based Cross Section Data to Represent Habitat Conditions for Riverine Resources at the Reach Level Page: 3
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Based on the recommendation of the NRC (2005), and consistent with Maidment et al. (2005),
the SAC (2009) led the development of the Hydrology-Based Environmental Flow Regime
(HEFR) Methodology. HEFR relies on a framework that quantifies key attributes of four
components of the flow regime intended to support a sound ecological environment. These
instream flow regime components are: subsistence flows, base flows, high flow pulses, and
overbank flows (SAC 2009). For each of these flow regime components, HEFR was designed to
assist in characterizing their attributes in terms of magnitude, volume, duration, timing,
frequency, and in conjunction with IHA or MBFIT, the rate of change. HEFR results are then
integrated with overlays of biology that include fisheries (i.e., physical habitat) and riparian
components as well as overlays of water quality and geomorphology. A description of the
ecological function of these flow components can be found in Richter et al. (2006), Richter and
Thomas (2007), TIFP (2008) and SAC (2009).
Physical heterogeneity of riverine systems influences species richness and abundance (Hynes
1970, Vannote et al. 1980, Ward 1989). Furthermore, in riverine systems, the physical habitat
structure (microhabitat and mesohabitat scales) is one of the critical factors that determine the
distribution and abundance of aquatic organisms. In general, as spatial heterogeneity increases at
the scale of aquatic organisms, there is greater microhabitat and hydraulic diversity that leads to
greater biotic diversity. This variability in physical habitat from the microhabitat to mesohabitat
scales is primarily derived from the physical processes of flow and sediment both within the
channel as well as the lateral connectivity of floodplain habitats. The diversity and availability
of these habitats are in turn maintained by variability in the flow regime and is a key process in
the evolutionary response of aquatic species life history traits that allow them to exploit this
variable and dynamic habitat mosaic. In many instances, the successful completion of various
life history requirements requires use of different habitat types. For example, spawning and egg
incubation may occur in riffles (turbulent velocities in conjunction with appropriate substrate
sizes); upon hatching, the fry move to the slow side margins of the stream, while non-spawning
adults may primarily inhabit deep pools. This variability in space and time of the habitat mosaic
directly (or indirectly) influences the distribution and abundance of riverine species as well as
overall ecosystem function (Poff and Allan 1995, Schlosser 1990, Sparks 1992, Stanford et al.
1996).
Because stream flow is one of the key factors that controls the temporal and spatial availability
of stream hydraulics (interaction of depth and velocity), substrate, cover, food, and, to a lesser
extent, temperature (e.g., Statzner and Higler 1986), stream flow within a given river system
controls the abundance and diversity of physical habitat and ultimately the diversity of species
that can exist. Ecological flow regimes are aimed at maintaining the natural diversity of habitats
(i.e., riffles may only represent seven percent of available habitat types) rather than the often
false assumption that flow regimes should optimize diversity. Optimizing habitat diversity is not
the same as maintaining habitat diversity, which is required to maintain ecological integrity of
aquatic ecosystems. One method of quantifying the effects of stream flow on riverine biota is to-3-
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Hardy, Thomas B. Evaluation of Gage Based Cross Section Data to Represent Habitat Conditions for Riverine Resources at the Reach Level, report, May 10, 2015; San Marcos, Texas. (https://texashistory.unt.edu/ark:/67531/metapth838905/m1/5/: accessed May 7, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting UNT Libraries Government Documents Department.