Bear Creek Habitat Assessment

Assessment of Physical Stream Habitat, Bear Creek, Arkansas

Joanna M. Reuter and Robert B. Jacobson

Introduction

We are currently working on a project to assess availability of physical stream habitat on a reach of Bear Creek within the National Park Service boundary of the Buffalo National River in Arkansas. By collecting data on the geomorphology and hydrology, we ultimately plan to assess sensitivity of physical stream habitat in Bear Creek to environmental change. In order to do this, we must first try to understand geomorphic and hydrologic interactions under present-day conditions. In particular, we would like to learn more about what magnitude flows are important in moving sediment and shaping the stream channel, and we are working to understand the relationship between the volume of flow through the stream and habitat availability (for example, the amount of pool or riffle area).

This web page presents our objectives, approaches, and progress. If you have questions or comments, please use the email address at the bottom of the page to contact us.

Setting

Bear Creek, which is located in the Ozark Plateaus Physiographic Province, flows northward for a distance of approximately 45 kilometers from its headwaters near the community of Welcome Home, Arkansas to its junction with the Buffalo National River near the town of Gilbert. The headwaters are in the rugged Boston Mountains, which are dominated by sandstone and shale bedrock formations. For much of its length, Bear Creek flows through a wide, flat valley that is heavily used as pasture land. This is the valley of the Boone formation, a highly permeable formation composed of soluble limestone and non-soluble chert. Closer to its mouth, Bear Creek flows onto older geological formations consisting of dolostone, sandstone, and limestone. Approximately two kilometers before flowing into the Buffalo River, Bear Creek enters the park boundary of the Buffalo National River, a unit of the National Park Service.

Our study reach is located within the National Park Service boundary, approximately one kilometer upstream from the mouth. Bear Creek is entrenched at the study reach; it is bounded by high alluvial terraces on the inside of the bend and by a bluff on the outside of the bend.

Physical Stream Habitat

By physical stream habitat, we mean the various combinations of water depth, velocity, and substrate that are available to the organisms in a stream ecosystem. Our approach to assessing aquatic habitat focuses on two key components:

Hydrology: As discharge changes, the amount of stream channel that is inundated changes, and the combination of depth and velocity at any particular location also varies. These habitat-sustaining flows determine what habitat is available on a day-to-day basis.
Geomorphology: Higher flows move sediment and rework the topography of the channel. Although these habitat-maintaining flows occur infrequently, they are important for the maintenance of physical stream habitat.

Habitat-sustaining Flows

We would like to quantify the physical habitat availability over time for the present day stream channel. In order to do this, we have to consider how habitat varies with discharge--a task for which we use a 2D hydraulic model, and how discharge varies over time--which is where gage records become important.

A brief description of this process follows, but for more information, a technical document is available online for a previous habitat assessment project using 2D modeling in the Ozarks.

2D HYDRAULIC MODELING

A two-dimensional hydraulic model uses the topography of the stream channel in combination with hydraulic parameters to calculate the depth and velocity that would occur at a set of points in the stream channel for a given discharge. (The velocity calculated for each location is depth averaged; that is, one velocity is calculated for each xy spatial location. This is why it is called a 2D model.)

After using field data for calibration and verification, the model can be run for a range of discharges. For each discharge, the model will produce maps of depth and velocity. A habitat classification scheme can then be applied to the map data. For example, areas with relatively deep, low-velocity water could be classified as pools, while areas with relatively shallow, fast-moving water could be classified as riffles. In this way, a relationship between discharge and physical aquatic habitat availability can be determined.

We have created maps of topography and substrate for the reach. These maps form the basis of the 2D hydraulic model at Bear Creek, which is currently being developed.

GAGE DATA ANALYSIS

The USGS and National Park Service operate gages which record the water level on Bear Creek. One gage (Bear Creek near Silver Hill) has real-time data that are available online from the USGS. This gage has been operating since January 1999, though it was moved to a new location in fall of 2001. Two additional gages, which are located at the upstream and downstream ends of the study reach, have been recording stage data since May 2001.

Several gages are operated on other streams in the Buffalo River basin and vicinity, and these may provide valuable information to supplement the relatively short gage record on Bear Creek. Although interpretation of other gage records must take into account factors such as basin size and bedrock geology, these gage records may assist in characterizing the magnitude and recurrence intervals for large flood events.

Habitat-maintaining Flows

The 2D hydraulic model is useful for assessing habitat availability given the current topography of the stream channel. However, the channel is shaped by the flows that pass through it, with high flows being particularly important. Because the magnitude of flow that is needed to transport sediment is difficult to assess, and the amount of sediment affected by a flood event is difficult to quantify, we are taking several approaches to get at the question of sediment transport.

CROSS-SECTION MONITORING

Thirty-one transects through riffles, races, pools, and glides were established and surveyed in detail during June 2001. The end points of the cross-sections were marked with rebar, so we can resurvey the cross-sections, thereby monitoring for erosion and deposition that occurred during the intervening time. We plan to resurvey all 31 cross-sections in December 2001 and spring 2002.

SCOUR CHAINS

Scour chains provide a method of assessing the depth to which bed load transport has occurred at a particular location. A piece of chain is implanted vertically into the stream bed and anchored in place. The chain is cut so that it is flush with the surface. After the occurrence of a flood event (ideally just one), the chains are dug out. If the chains are bent over, this indicates that sediment transport did occur. The depth to which movement of bed material occurred can be determined, as can changes in the bed surface elevation.

In September 2001, we installed a total of 18 scour chains in three glide/riffle pairs. As of November 2001, we are still waiting for a large storm event.

PAINTED ROCK EXPERIMENTS

Painted rock experiments involve putting marked rocks in the stream bed, waiting for a sediment transporting event, and finding the rocks again to determine if and how far they have moved. This is another qualitative method of addressing the sediment transport issue.

Painted rock experiments will be performed during winter and spring of 2002 if flow conditions allow.

SEDIMENT TRANSPORT CALCULATIONS

In addition to observational methods to assess sediment transport, several mathematical approaches are available to make predictions about when sediment transport will occur. The size of sediment in the stream is of course one of the factors that is used in such calculations. We have data on sediment size distribution at Bear Creek from pebble counts, in which measurements were made of 100 randomly selected rocks in the channel. Pebble counts have been completed for all 31 transects at Bear Creek.

Products

A model for assessing sensitivity of habitat to altered flows. Once we have developed the 2D model, we can analyze the data in terms of altered hydrologic conditions in order to assess the sensitivity of habitat availability to a change in conditions.

A better understanding of conditions necessary to transport sediment. The extent to which we will be able to assess sediment-transporting events is dependent on weather conditions and is therefore limited by the two year time frame of this project. However, by taking multiple approaches, we hope to maximize our chances of obtaining meaningful data.