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Guidebook to the Geology of Travis County

Field Trip No. 1: Shoal Creek, Austin, Texas

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I.   To the Teacher and/or Field Trip Leader
II.  Students' Introduction and A Brief History of Shoal Creek
III. The Tour


TO THE TEACHER AND/OR FIELD TRIP LEADER

This field trip is designed to be a two to two and one-half hour field event followed up with a "post lab" classroom discussion and/or evaluation session. Your students will learn more from this trek if you "pre lab" the section, students' introduction, concentrating particularly on the terms listed and concepts relating to the terms. The students' introduction also has an assortment of suggested questions designed to "cue" your students to look for certain aspects or relationships that could be overlooked by making the field trip a "show and tell" experience. Conditions do get hectic and hurried on a field trip, especially with eighth graders; therefore, the better prepared your students are beforehand, the more they will see and remember afterward.

          Two additional points:
Although a "sketch map" is provided for tour routing and suggested stopping points, several of these points offer limited parking for an auto caravan or a large bus. You should survey the route beforehand to determine which points will provide the best learning experiences for your students.
Wooten Park (Stop #6) has plenty of sights to offer, including a neat place to break for picnic lunches.


STUDENTS' INTRODUCTION

The Shoal Creek field trip is your opportunity to examine an urbanized creek from its headwaters to its end at Town Lake. When you complete the tour, you will have a good idea of the changes man makes on a creek when he builds homes, roads, streets, and bridges in the creek watershed. A Glossary of terms and a list of suggested questions are provided to refresh your memory on certain features of landform. Please refer to the questions on this page and to the Glossary found on the back pages.

When you complete this tour, you should be able to explain in your own words the answers to the following:

1. List three or more activities of man that have made Shoal Creek more likely to flood than when it is in its natural state.

2. The natural springs at the headwaters of Shoal Creek are almost dried up. Can you think of some reasons for this?

3. What effect has "urbanization" had on:
a. the speed of water runoff?
b. stream bed sedimentation?
C. soil stability?

4. What do you think should (and should not) be built on Shoal Creek's flood plain?

5. What could be done to make Shoal Creek a safer part of Austin?

6. What are the advantages and disadvantages of making Shoal Creek a "storm drain channel"?

7. Would Austin have any problems installing storm sewers to take the runoff load from Shoal Creek? List your ideas on these problems.

8. Would deepening the creek in flood prone areas solve the problem? Explain your answer.

9. What causes the green slime in pools and slow-moving areas of the creek? (Note: most natural streams have little or no slime).

10. List all of the types and locations of stream pollution you noted during the field trip.

11. The City of Austin has built the main sewer lines along the creeks whenever possible. Can you give at least one advantage and one disadvantage of this practice?


Brief History of the Shoal Creek Area and Austin

Not very long ago in geoloaic time, Austin was part of a huge inland sea which stretched from Texas to parts of Illinois, Idaho, and the Dakotas. The very land you are standing on was at times as much as 100 meters (300 feet) or more below the surface of the water! Yet, there were also times when the waters were shallow and dinosaurs waded around in our region. Pilot Knob, an ancient volcano off the Locl,,hart Highway south of Bergstrom Air Base, rose above the sea, belching smoke, ash, and lava. Actual beaches formed around this volcano (which later became inactive) and looked from the air like one of the many volcanic islands (called atolls) remaining in the Pacific Ocean today.

The land was not still in that time, nor is it still todav. High land is worn low and flat, and old flat land is raised upward to form new hills or mountains. Austin is now what was once the bottom of the sea. What we are concerned with is what happens to this sea floor when it becomes high ground and when wind, rain, and man begin to work on it.

To really understand how a creek "operates", you must understand that it takes a lot of land to shed enough water to get a stream going in the lowest parts of the area. When it rains, a part evaporates and returns to the air. The remainder, which flows downhill on the surface of the ground via small gullies, or via city storm drains, is the main water supply or the stream. If there is enough water, the stream will travel toward lower points, often joining other streams until it gets to the lowest point possible -- a lake or the ocean. Often, during very dry weather, a stream will not have enough water coming in from its watershed, and it will become a dry, rock stream bottom. During periods when a great quantity of rain falls, the stream will rise and swell with swiftly moving quantities of water just like a medium-sized river. And when it really rains -- well, you will see what this can cause during your trip.


THE TOUR

STOP #1
North access road to U.S. 183 on the east side of the Missouri-Pacific Railroad watering stop.

The scene facing north is the headwaters of origin of Shoal Creek. This field of tall grasses and few trees doesn't look like much, but the waters begin to move downstream from this field and from the Balcones uplift to your left.

Once, there were welling springs in the field area. Steam locomotives of the Missouri Pacific drew their water supply from these natural springs. A few of the springs still produce surface water, but as you can see, the water either evaporates or is absorbed by the ground before it can get to where you are standing. Of course, if it has rained enough recently, you should see some water running by your position.

Why do you suppose there would be springs in and near Austin? Can you think of other natural springs in and near Austin? It just so happens that these springs follow all along the Balcones fault from north of Dallas to south and east of San Antonio. A natural underground aquifer or "waterpipe" was broken by the uplift or fracture along the Balcones Escarpment many years ago, and the waters found their way to the surface by following the fault upward.

You will notice that the high ground to the left, the Balcones Uplift, is covered with trees, but the field ahead and to the right of your position is mostly covered with grass and shrubs. This points out that there is a difference in soils and the rock underlying those soils between the highland and the adjoining lower land.


STOP #2
Proceed from the access road to Shoal Creek Boulevard (across and south of Hwy. 183).

You will notice the sudden widening and deepening of the creek. This area is on the normal creek flood plain, so the creek was straightened, widened, and deepened to handle the immense rush of water draining off the highway and adjoining commercial buildings and large paved parking lots whenever the rains are intense. A good rain will fill this canal to the point of overflowing in ,less than two hours, although the water moves downstream very rapidly. Can you imagine how much water that is? If you look along the banks, you will see several large, concrete pipes jutting out into the creek from the banks. Some of these pipes are over three feet in diameter. They are storm drains which collect water from your streets and carry it underground until it comes out somewhere along a stream or creek. These drains really move the water efficiently and quickly, and seldom does a street or lawn fill with water even in the heaviest rain. The trouble is that the poor creek gets all that water so fast and so heavy that sometimes the creek can't handle it and flooding results.

What effects do these storm drains have on Shoal Creek besides flooding? Do you see any effects of the storm drain on the opposite bank -- any signs of erosion? -if not, keep the storm drains in mind as we proceed downstream.


STOP #3
Spicewood Springs Road and Shoal Creek

This small tributary stream is one of the first natural "feeders" into Shoal Creek. Notice the deep dip in the road (Shoal Creek Boulevard) to allow flood waters to flow over the road. It was very near here that a motorcyclist was drowned in June 1972. Can you see why? Now go to the bridge and just look around. The underpass of this bridge is much too small for the amount of water now pouring past this point during flooding. When the bridge was built some years ago, this passage probably was able to handle the stream flow. Now, the water simply backs up until it can rise over the bridge! Look at the guard posts being undercut by this overflow. Now note the stone pillar across the street by the apartment. Check the erosion on the roadback. This is a very dangerous area to be in during a heavy rainstorm.


STOP #4
Return to Shoal Creek Boulevard and turn left (south) -- arrive at Northwest Park.

Enroute to Stop #4 you no doubt noticed that there are many nice homes along the creek. Almost all of these homes are located on the creek floodplain. Several of them have experienced frequent flooding. This park (northwest corner) marks the spot where the deepening and widening of Shoal Creek ends. From here on downstream to Wooten Park, the narrow, meandering Shoal Creek occupies its natural bed. When all of the rushing water gets to this point, the narrow banks, trees, and rushes slow the stream down, and cause it to spread over lawns and homes that are within your view.


STOP #5
Return to Shoal Creek Boulevard via Alberta and Daugherty and Greenlawn, retracing the route from the Boulevard. Shoal Creek Boulevard at Hunt Terrace.

This is the area which experiences flooding every three to five years. You can imagine that as upstream development continues, the flooding here will become more severe and more frequent. If we could plan the use of this area all over again, what do you think we should do with it?


ENROUTE TO STOP #6
Northland and Shoal Creek

The road engineers wanted to put in the less expensive shorter bridge, so they narrowed the banks at this point. Looking upstream, you can notice the shallow banks where the backed-up waters flood.

Hancock Drive and Shoal Creek

This bridge was designed to handle the original 10 year flood calculation (before urbanization), but it is now completely inadequate and, therefore, frequently floods.

45th Street

Downstream, the creek channel is naturally deep and is not flood-prone except for the west bank housing area which is too close to the channel.


STOP #6 - WOOTEN PARK

You are looking at what could be called a river channel in some places. Upstream, urbanizatiion has increased the water flow so much that Shoal Creek is downcutting about two inches a year at this point. Those hugecobbles were rolled down from the Shoal Creek Hospital and Medical Tower construction sites upstream. A few of the larger ones probably weigh nearly 1000 pounds. Can you imagine the water force needed to move them? What would such a force do to a brick house? Have you bothered to keep a count of the number of storm drains you have seen since we began the tour?


LUNCH BREAK!

This is normally the conclusion of the tour except for small groups, since further points downstream are in very congested areas. Parking is available at the Municipal Power Building, and your group could find an interesting terminal view of Shoal Creek as it empties into Town Lake.


OPTIONAL STOP #7
Drive south on Lamar Street and turn left at 5th Street. There is limited parking space just west of the bridge over Shoal Creek on the right (by a used car dealer).

If your group is small and manageable, you might let them follow the creek to the north side of the power plant (West Street and First) where you can pick them up. This mini-hike will be self-explanatory for the following features:

1. Several severe slumps and/or bank failures.

2. Severe erosion at 90 degree right turn in the stream at Third Street area.

3. Raw effluents entering the stream from both a large storm drain and from an area very close to the power plant.

Although you have made several stops during the last two or three hours, there should be many things which stand out in your mind as being changes caused by man. As you return to your classroom or home, take a pencil or pen and a sheet of paper. Together with a friend, make up a list of all the things you can remember that man has done to Shoal Creek to change its natural condition.

MAP 7-150K


On to Field Trip #2... change its natural condition.

MAP 7-150K


Back to the Guidebook  :  On to Field Trip #2...