Hydraulic Modeling of Ozarks Stream Habitats

Section 1:  Introduction and Background

Physical stream processes and resultant habitats define the template upon which in-channel communities evolve and are maintained.  Stream conditions such as depth, current velocity, substrate, and temperature combine to form unique habitats which facilitate the growth and survival of particular species and species assemblages.   Lobb and Orth (1991), for example, found a preference for particular physical habitats among the life stages of warm-water stream fishes.  Young fish prefer macrophyte beds and edgewater habitats for nursery areas, juveniles prefer channel margins for foraging and avoiding predation, and adult life-stages prefer pool, edgewater, and large woody debris habitat.  While biotic effects such as competition and predation may also shape the size and structure of a community, many stream ecologists consider physical habitat features to be the major determinants of stream community potential (Schlosser, 1987; Plafkin and others, 1989) (Figure 1.1).   Generally an increase in the diversity of physical habitats in a stream correlates with an increase in species diversity (Gorman and Karr, 1978; Schlosser, 1987; Jeffries and Mills, 1990).

Recognition of physical habitats as community templates has led to efforts to map, inventory, and model habitat distribution as a predictor of organism abundance and/or diversity.  These efforts are complicated by the dynamic nature of streams.   Lotic habitats change both over the short term as channel hydraulics respond to the rise and fall of discharge, and over the long term as channel morphology adjusts to changes in basin hydrology and sediment load. Both scales of temporal fluctuation present challenges for stream ecologists and natural resource managers. First, hydrologic fluctuations make it difficult to quantify the distribution and abundance of different habitats—the extent of a pool for example, may increase twofold as a rising discharge expands channel area. Second, regional changes such as climatic shifts or watershed urbanization may alter basin hydrologic characteristics or sediment load. Changes in either one of these factors can ultimately affect stream communities by changing habitat characteristics and extent.

Given their spatial and temporal varability, two approaches are commonly used to inventory aquatic habitats.  The first is field based and involves detailed surveying of depth, velocity, and substrate distributions along cross sections throughout a channel reach (e.g. Hogan and Church, 1989; Lapointe and Paine, 1996).   The second approach involves hydraulic modeling such as the Physical Habitat Simulation System (PHABSIM) (Millhous and others, 1989; Bovee and others, 1998) and uses a base-line of field data and one-dimensional hydraulic models to simulate depth and velocity distributions for conditions that are not measured directly.  In both cases, the suitability of the hydraulic conditions are usually evaluated on a species or life stage specific basis--hydraulic preferences are identified for an individual species and then the area of habitat within those conditions is quantified.  For example, PHABSIM quantifies the Weighted Usable Area of habitat available for the range of modeled discharges (Bovee and others, 1998).

Both field and modeling approaches have been used successfully to quantify habitat availability and sensitivity to discharge fluctuations. For example, as part of the Instream Flow Incremental Methodology (IFIM), PHABSIM, has been used extensively to anticipate and describe biological effects of flow regulation on economically important or endangered species (e.g. Bovee and others, 1998).   Field and modeling based habitat inventory methods also have limitations.  For example, both methods usually collect and display data along cross sections leaving intermediate channel zones undescribed.  Field based approaches are also hindered by the time and labor intensive nature of data collection for multiple discharges and by safety issues involved in sampling higher discharges.  Modeling approaches overcome some of these concerns but introduce limitations inherent in the chosen model.  For example, PHABSIM first relies on one-dimensional models to estimate water-surface elevations along a channel center-line and then uses that and other information to extrapolate flow velocities and depths along perpendicular cross sections.

As a means to overcome these hydraulic limitations, more recent modeling studies have moved toward two-dimensional finite element hydraulic models (e.g. Waddle and others, 1997).  These models calculate depths and velocities for the discharge of interest across a topographic mesh defining a channel reach.  One of the main advantages of two-dimensional models is this data coverage: element nodes blanket the channel reach and can be spaced at an interval suitable for detailed habitat studies in the stream of interest.  However, two dimensional models maintain some of the limitations of modeling systems such as PHABSIM: they determine vertically averaged flow velocities and assume a static bed geometry.   This means that these models do not depict vertical velocity gradients and are most appropriate for modeling discharges below that required for incipient stream-bed motion.  Repeated surveys of channel geometry remain the best way to evaluate the effects of changing channel morphology on habitat distribution and abundance (McKenney and Jacobson, 1996; McKenney, 1997).

This document describes an approach to habitat inventory using the two-dimensional hydraulic model RMA-2.  As part of the Ozarks Stream Geomorphology Project (Appendix A.1), modeling was carried out for the Ratcliff Ford reach of the Jacks Fork River in the Ozark Highlands of Missouri (Figure 1.2).  The modeling objective was to assess the sensitivity of in-stream physical habitats to climatically induced hydrologic variations.  Rather than assess habitat availability for a specific species or life-stage, we have chosen to inventory the distribution and abundance of hydraulic habitat units.  A field based classification system and a Geographic Information System (GIS) were used to classify hydraulic modeling results into five physical habitat categories based on Froude number and depth.  We chose this approach as a means to assess community implications of hydraulic variability in a river system where the primary conservation goal is the maintenance of an extremely high aquatic biodiversity . By developing a modeling methodology and analyzing habitat dynamics, this paper is a first step toward linking physical habitat changes with biotic responses.   We hope that this methodology will facilitate biological studies of physical habitat requirements and the biological implications of hydraulic variability.

Statement of Purpose

This paper investigates the sensitivity of stream physical habitat to changing hydrologic regimes. The two-dimensional hydraulic model RMA-2 was used to analyze changes in physical habitat availability created by climatically driven variations in stream discharge. Our goals were twofold:

1. To develop methods for aquatic habitat inventory using hydraulic models and historical stream-gage records.
2. To gain insight into the sensitivity of Ozarks stream physical habitats to changing hydrologic regimes.

Study Area

The Ozark Highlands (Figure 1.2) is a rugged, montane region surrounded by modest topography and intensive agriculture. The Ozarks maintain contiguous, wild habitats that are rare and highly valued in the mid-continent of North America. The karst drainage system that underlies most of the Ozarks has created a wide range of terrestrial and aquatic environments, from upland glades to sinkhole marshes, and from spring-fed coldwater streams to highly variable warmwater streams. The juxtaposition of habitats and outstanding water quality support an extremely high biodiversity. In recognition of the fish and wildlife resources of the region, Federal and State agencies have purchased and manage extensive tracts of land. Included in these lands are the Ozark National Scenic Riverways a river corridor park administered by the National Park Service.

The Ozarks are also highly valued for mineral, timber, agricultural, and recreational resources. Historical and present-day land uses have affected fish and wildlife resources of the Ozarks, and it can be expected that land use will continue to present potential stressors. Expanded lead mining, intensive timber harvest, and conversion of timberland to grazing land are current land-use concerns with the potential to disturb Ozarks streams. In addition, the Ozarks are particularly susceptible to climatically driven stressors because thin soils and steep slopes provide little capacity to mitigate climatic extremes. Moreover, the interplay of Gulf, Pacific and Arctic air masses creates a zone of climatic tension over the central U.S., whose balance is likely to be affected by global-scale climate change.

This project focuses on the sensitivity of the Jacks Fork River to environmental changes that alter stream-flow conditions. The 1160 square kilometers (km²) Jacks Fork basin is representative of many watersheds in the Ozark Plateaus in south-central Missouri (Figure 1.2). Elevations in the study area range from 220 to 470 meters above sea level and mean annual precipitation is 1000-1200 mm (Vineyard and Feder, 1974). The Jacks Fork is incised into Paleozoic chert-rich carbonates, sandstones, and shales; locally, these lithologies control valley width (100 to 600 meters) and create steep bedrock bluffs. Chert gravel from the carbonates dominates bedload in the Jacks Fork and extensive gravel bars characterize most reaches. As is typical in the Ozarks karst terrain, spring flow from an extensive cavern system maintains the river’s base flow.

Detailed surveying and biological sampling have taken place at three study reaches on the Jacks Fork; from upstream to downstream the reaches are named Fox Farm, Ratcliff Ford, and Burnt Cabin (Figure 1.3; Appendix A.2) (McKenney and Jacobson, 1996). Hydraulic modeling in this study was conducted for the middle reach at Ratcliff Ford which drains 422 km² and has a channel gradient of 0.20% (Table 1.1). The Ratcliff Ford reach is part of the most confined portion of the Jacks Fork River, the valley is bounded by steep, dolomitic bluffs and is 100 meters wide (Figure 1.4 and Figure 1.5). The reach is 535 meters long and includes a pool-riffle-pool-riffle-pool sequence.

Table 1.1.  Hydrologic characteristics for the Jacks Fork River at continuous streamflow gaging stations and at the Ratcliff Ford study reach.  Refer to Figure 1.3 for gage and site locations.

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