Project Purpose

 The project objective is to identify potential segments of Texas waterways to be classified as "National Wild and Scenic Rivers". According to the Wild and Scenic Rivers homepage of the National Parks Service, only one river reach in Texas currently holds this designation. Authorized in 1978, this river reach is "a 191.2-mile strip on the American shore of the Rio Grande in the Chihuahuan Desert…It begins in Big Bend National Park and continues downstream to the Terrell-Val Verde county line."

This initial screening for potential river segments will be based on the following criteria:

1. river segments should be free of upstream dams, or significantly distance from upstream dams,
2. segments should drain a large enough area so that significant water is present, and
3. segments should pass through scenic countryside.

The approach for identifying the river segments free of the affects of upstream dams will be to subdivide the 13 major river systems in Texas into subwatersheds using dam locations as outlets. The number of dams within each watershed will then be counted and the watersheds with the fewest dams will be selected. The next step will be to identify the river segments within each watershed that drain a significantly large area. These segments will be further queried to find those that flow through forested regions.

Data Sources

The data used for this analysis were obtained from the dataset: "Hydrologic Modeling in Texas Using GIS", August 1997, compiled by Francisco Olivera, Juling Bao, and David Maidment, Center for Water Research in Water Resources, University of Texas at Austin. A number of data layers from this source were used including: DitchDEM, Flow Direction, Flow Accumulation, Reservoirs, Stream, Land Use and a raster map of the study region. Additionally, several steps developed in the HEC-PREPRO hydrologic processing module were used to analyze this data. More information about the data can be found here. More information about the data processing module can be found here and here. Information about specific coverages used and new sets generated can be found in the data dictionary.

Project Outline

 The following tasks and procedures were used to complete the project:

TASK 1: Identify Regions where Dams are Infrequent

• Procedure 1: Create a point shape file of dam locations in Texas
• Procedure 2: Set Parameters for the PRE-PRO model 
• Procedure 3: Create Sub-Watershed based on Dam locations
• Procedure 4: Count Number of Dams in Each Sub-Watershed
• Procedure 5: Select Sub-Watersheds with minimal dam influence

• Procedure 6: Identify River Segments that Drain Threshold Area
• Procedure 7: Extract River Segments within Sub-Watersheds from TASK 1

• Procedure 8: Extract River Segments that flow through scenic areas
• Procedure 9: Displaying the Data

Project Procedures

Procedure 1: Create a point shape file of dam locations in Texas Top

It was important that dam locations overlay on the stream grid coverage for the PRE PRO module to work properly. To ensure a good match, a new point shape coverage was created using the following procedure:

1. Add the reservoir (txdata/reserp) and stream themes ("da_ok" from the Hydrologic Modeling dataset) to the PRE-PRO project shell
2. Select NEW THEME from the THEME menu. Select "Point" as the feature type and name the new theme "Dams.shp".
3. Using the "Add Point" tool to locate the dams at the downstream edge of reservoirs on the stream grid. In cases where reservoirs do not fall along the stream grid, points should still be added at the downstream edge of the reservoir. This will allow for a full accounting of the number of dams in a sub-watershed.
4. Once all points are located, choose "Stop Editing" from the "Theme" menu and save your edits when prompted.

Procedure 2: Set Parameters for the PRE-PRO model Top

In order to use the PRE-PRO package to create new watersheds based on the dam locations, several themes must be present in the PRE-PRO project shell. Some of these are already present in the "Hydrologic Modeling" dataset and others were generated. The table below summarizes the functions that were used in the PRE-PRO module along with their associated data layers. The color of the text identifies the origin of the data layer:
 

• ditchgmf (a burn DEM coverage for all of Texas and parts of New Mexico, Oklahoma, Arkansas, and Louisiana, renamed-DitchDEM),
• txfdmod (a flow direction grid for the study area generated from the burn DEM, renamed-FlowDir),
• txfamod (a flow accumulation grid for the region generated from the flow direction grid, renamed-FlowAccum), and
• da_ok (a stream grid generated from the flow accumulation and flow direction grids with a threshold drainage area of 10 square miles, renamed-Streams)
• Outlets13 (a grid coverage of the 13 major river outlets in Texas, create by converting Outlets13 point file to grid with same extent as txfdmod)

Using the HEC-PREPRO Tools, the Links theme was created by the following procedure:

1. From the HEC-PREPRO Hydro Menu, select the Stream Segmentation function
2. Choose as input coverages, FlowAccum and Streams
3. Name the output theme, Links

Procedure 3: Create Sub-Watershed based on Dam locations Top

The next step is to use the Add Outlets and Sub-Watershed delineation tools to define watershed regions upstream from dams. The Add Outlets function modifies both the Links and Outlets13 grids. Additional outlets are added to the Outlet grid from a user defined point shape file. The Links grid—a segmented version of the Streams grid—is also modified by the point shape file. The new modified Outlets grid was used to run the Sub-Watershed Delineation function. To complete this procedure, the following steps were used:

1. Select Add Outlets from the Hydro menu.
2. Choose as input coverages, Links, Outlets13, Dams.shp, Streams, FlowDir
3. Name the output themes, ModOutlets, ModLinks
4. When function is complete, select Sub-Watershed Delineation from the Hyro menu
5. Choose as input coverages, FlowDir and ModOutlets
6. Name the output theme, Watershed

When this procedure was completed, 79 sub-watersheds were created. Of these only 66 were associated with a "dam outlet". The remaining 13 were associated with the original outlets of the major river systems of Texas. Closer inspection of the resulting watersheds revealed that dams along the main channel of rivers were favored over those on tributaries. It would be impossible this process to create a one to one relationship between a "dam outlet" and a sub-watershed. Additionally, no information about the presence or absence of a dam immediately upstream of a watershed was generated. These two drawbacks to the use of HEC-PREPRO to meet the first criterion defined for this project lead to the following procedure.

Procedure 4: Count Number of Dams in Each Sub-Watershed Top

The delineation of sub-watershed to locate "Wild" or undammed river stretches was only partially successful. Only 66 sub-watersheds from a total of 196 dam locations were created or roughly 33 percent. As a result there was not a one-to-one relationship established between sub-watershed and dam. This relationship would enable the selection of watersheds based on the size of watershed or the length of the flowpath upstream. The limited success of the approach was that a number of watersheds were created that contained only one dam at the outlet and were not downstream of another watershed. The following procedure was completed to count the number of dams in each watershed to limit the search to these watersheds:

1. Convert the Watershed grid to a shape file called Water79.shp
2. Convert the Dam.shp theme to a grid theme called Damgrid
3. Select the Water79.shp theme
4. Select the Summarize By Zone tool from the Analysis menu. This tool is used to summarize the attributes of sub-areas or zones of a grid coverage. A shape file first selected to define the zone that will be summarized in the grid. This procedure will query the Damgrid based on the regions established by the Water79.shp theme.
5. Choose "Grid Code" in the Define Zone By window. This links the identification number of each zone with the summary statistics that will be generated.
6. Choose Damgrid as the theme to be summarized.
7. The output of this analysis is a table including a field called "Count" which has totaled the dams in each watershed.
8. In the Table window, join the new "Summary of Damgrid by Water79.shp" table to the Water79.shp attribute table. Use "Grid Code" as the common value.
9. In the View window, the legend of Water79.shp can be modified to color code the watershed by "Count"—representing the number of dams in each region.

Procedure 5: Select Sub-Watersheds with minimal dam influence Top

From inspection, the Water79.shp file shows graphically which regions of Texas are most dammed. Those lightly shaded have only one dam, while the darkest have 13-15. No information is associated with the themes about the number of dams upstream or their distance upstream. If desired the "Wild" watershed could be selected manually, by looking for upstream/downstream dam relationships. The following procedure was used to "Wild" watershed:

1. Use the query tool to select only those dams with one dam (i.e. Count=1). Twenty-one watersheds were found to have only one dam. These consisted of a few that were downstream of another watershed.
2. Convert selected to SelectWater79.shp

Procedure 6: Identify River Segments that Drain Threshold Area Top

Once the "Wild" watersheds have been created in Procedure 1-5, the river network to be queried needs to be established. Under criterion 2, only rivers that have a significant amount of flow should be used in this exercise. Its time to "separate the rivers from the streams". The amount of flow in each river segment is going to be roughly related to the amount of area that that segment drains. The values in the flow accumulation grid were used screen out those stream segments that don’t receive significant flow. From several inspections, the threshold value that separates a river from a stream was determined to be 256860. The following procedure was used to remove the streams:

1. Select the FlowAccum theme
2. Selected the Reclassify tool from the Analysis menu
3. Set the options to view the values by 64 equal interval categories.
4. Reclassify the lowest range of values (0-256860) as "No Data".
5. Select Vectorize Streams from the Hydro menu
6. Choose as input themes, Reclass1, FlowDir
7. Name output theme, Rivers

Procedure 7: Extract River Segments within Sub-Watersheds from TASK 1 Top

With the river network establish and the "Wild" watersheds selected, the union of the two will produce the ‘Wild" River. The following procedure was used:

1. Make the Rivers theme active
2. Select the Select by Theme option from the Theme menu
3. In the query window, choose "Are Contained Within" and the Water79.shp theme. This criterion will select only those River segments that are within the "Wild" watersheds.
4. Convert resulting selected arcs to a new shapefile called WildRivers.

Procedure 8: Extract River Segments that flow through scenic areas Top

With the Wild River segments established, the next step is to establish those portions of the river that flow through scenic countryside. A Qualitative decision was made to equate scenic areas with forested areas—more specifically those with an Anderson Land Use code of "4". The following procedure was used to identify these stretches:

1. From the Land Use polygon coverage in the "Hydrologic Modeling in Texas" dataset, query the "Level 1" field for those polygon with a value of "4"
2. Convert the selected polygon to a shapefile named Scenic and second coverage was made for those areas with Level 1 value of 3 (Rangeland)
3. Make the WildRiver theme active
4. Choose Select by Theme option from the Theme menu
5. Set the options to "Intersects" and "Scenic"
6. Convert the selected WildRiver arcs to a shapefile called WildScenicRivers

Procedure 9: Displaying the Data Top

The third criterion has been met and the WildScenicRivers coverage created. The only remaining task is to identify exactly what rivers were selected and how to find them. The following procedure was used:

1. Add the USGS theme from the dataset "Hydrologic Modeling in Texas" to the project
2. Open the USGS theme and overlay the Rivers, WildRivers, WildScenicRivers and SelectWater79.shp themes.

The final output of Procedures 1 through 9 was three river segments in Texas:

1. The Llano River in Mason County and Llano County
2. The Nueces River below Carrizzo Springs in Dimmitt and LaSalle County
3. The Pecos River separating Crockett County from Pecos and Terrell County

The three river segments selected are reasonable candidates for further investigation. It is important to note, however, that two of them are downstream of dams (Nueces and Pecos). This occurred because the watershed approach to identifying wild river segments did not account for upstream dams immediately upstream. This problem was the result of a limit of the problem solving approach. An alternate approach that might yield better results would be to analyze the river network using an Avenue script. This approach would generate information about upstream and downstream network relationship and calculate flow distances. An additional portion of the project that could be refined, is the inclusion of more dams in the analysis. The USGS listed 301 dams in Texas, only 197 were included here based on the reservoir locations.

Created 5/1/98, Last modified 5/7/98, djs dscott@mail.utexas.edu