CAUSES FOR CHANGE IN STREAM-CHANNEL MORPHOLOGY

Transcription

1 CAUSES FOR CHANGE IN STREAM-CHANNEL MORPHOLOGY Chad A. Whaley, Department of Earth Sciences, University of South Alabama, MobileAL, The ultimate goal of this project was to determine the causes for change in stream channel morphology. With the use of Geographic Information Systems, a horizontal profile of Spring and Moore Creeks was created. The examination of both these profiles can help us understand and analyze the causes of erosion, sand deposits and sediment transport by measuring elevation changes. By comparing the elevation changes from the profile with the actual land, it can then be determined if changes in the stream channel are due to human or natural causes. Findings supported a conclusion that changes in elevation along both creeks were due to human activity. Long-term adjustments can now be made in relation to drainage changes or base-level changes through the use of horizontal profiling. This technique can be useful for predictions of up to several decades, which can in turn provide valuable information to the surrounding community and Dog River Clearwater Revival. Keyword: stream, channel, morphology, Geographic Information Systems, horizontal profiling, sedimentation. Introduction: Covering a total area of approximately ninety-five square miles, The Dog River Watershed (DRW), encompasses most of Mobile s streams and creeks. Nearly 60% of the watershed lies within in the city s boundary. Dog River serves as not only a drainage way for Mobile, but also as recreational waters for surrounding residents (ADEM, 1994). As with any watershed, there are major concerns that affect people and the environment in a variety of different ways. Erosion, runoff, sediment transportation, deposition are just a few of the concerns. To better understand the causes of these watershed problems, research has been conducted on changes in stream channel morphology within the Dog River Watershed. Geomorphology is the study of landforms including the origin, evolution and the processes that form them. These processes are each influenced differently by climate, ecology, and human activity. Present day structures are studied not only to help us

2 understand the geological features we see today and the forces that created them, but also to help us understand the features from previous ages that we see recorded in geological history (Schumm 1977). River morphology is the field of science dealing with changes of river form and cross-section shape mainly due to sedimentation and erosion processes. River morphology is a complex subject, which can be subdivided into several different areas that are often related to each other. These areas include: overland flow and channel flow, drainage systems and channel networks, stream channels, stream flow, and stream stages, stream discharge and basin area, horizontal profiling and profile development, stream erosion and transportation (Schumm 1977). The focus of this research is the aspect of horizontal profiling on stream channels. A good way to understand stream channel morphology is to conduct a horizontal profile of the actual streams. The monitoring of the profile can help us to understand and analyze the causes of erosion, sand deposits and sediment transport by measuring elevation changes across set transects on a stream over time (Russell, 1895). Streams, creeks and rivers are all landforms that are subject to rapid change in channel shape and flow pattern. Water and sedimentation build up can determine the dimensions of a stream channel (width, depth, meander wavelength, and gradient). What is the significance? The physical features of stream channels reflect the magnitude of water and sediment discharges. In the absence of different hydrologic data, an understanding of stream morphology through horizontal profiling, can help to conclude if any environmental changes have taken place (Chang, 1988). When looking at changes in stream channel morphology, people often wonder

3 why these changes have taken place. Are the changes due to human or natural causes or even both? Land use changes, such as land clearance, construction, farming and forestry can all have an effect on streams and rivers. Climatic events such as heavy rainfall, flooding, and tectonic activity are believed to be the primary cause for morphology changes. Assumptions are also being made that human activity is to blame for changes in stream morphology over time. Research Question: It has been my research goal to examine changes in stream morphology by horizontal profiling and to assess if the changes were associated with management, topography or other factors. Are there similarities and/or differences in the morphology of nearby streams? Is there some natural or human cause for erosion, sedimentation transport or deposition in a certain area of the streams profile? What are the causes? Through the use of Geographic Information Systems (GIS), I will inform the surrounding community that horizontal profiling can prove beneficial to the assessment of the causes and effects regarding changes in stream channel morphology. Methods: Geographic information system (GIS) is a computer system capable of integrating, storing, editing, analyzing, sharing, and displaying geographically-referenced information. In a more generic sense, GIS is a tool that allows users to create interactive queries (user created searches), analyze the spatial information, edit data, make maps, and present the results of all these operations(goodrich 2007). The goal of this study was to derive predictive relationships for stream channel characteristics using a highresolution ArcInfo GIS database.

4 I chose Spring and Moore Creeks, located in the Dog River Watershed, to examine in my study. Utilizing GIS technology, I investigated the morphological relationships of the two creeks previously mentioned. The City of Mobile offers a wide variety of shape files that can be used to produce various types of maps using GIS software. Obtaining the data for Dog River Watershed area, streams, water bodies, major roads, 2-foot contour lines and an aerial photograph of the City of Mobile was the first step in the process. The area of Spring and MooreCreeks, Figure 1, were selected from the watershed data file. With this specific area, I created a map using ArcMap, a GIS software with the capabilities of producing, editing Figure 1: Dog River Watershed, Mobile AL. Source: Dog River Watershed Website 2007 and analyzing maps of all varieties. I displayed streams, major roads, bodies of water and the aerial photograph of the city designated area. To measure or analyze any elevation changes, a surface layer had to be created in order to make observations and derive

5 conclusions. Using the topographic to raster image function in ArcMap spatial analyst, a surface layer was created from the 2-foot contour file (2002) of the Spring and Moore Creek s vicinity. With an elevation surface now established on a map, the horizontal profile of the two creeks could then be created. The length of Spring and Moore Creeks were then digitized from the head waters (beginning) down to the mouth (end) of both creeks. Horizontal profiles of Spring and Moore Creeks were then established. Studying the completed profiles and analyzing the creeks elevation changes, I then measured distances along both creeks in the map so that reference points could be made on the profile graphs. Once reference points were accounted for, I then chose two sites along each creek where there were obvious changes in elevation. I went out in the field to the actual creeks and compared and contrasted the data that was presented to pin point the causes of change in morphology whether natural or manmade. Results: After creating a horizontal profile for both Spring and Moore Creeks and doing field observations, I came to the conclusion that the most drastic elevation changes in the profiles were due to human activity or manmade causes. From the horizontal profile of Spring Creek, Figure 2, two sites on the graph were chosen that were of interest. The first major elevation change was simply due to a major road, Cottage Hill, being in the path of the creek. Also located here was a large pond which was only visible from an aerial photo of this particular place, (Figure 3). The elevation gradually decreased by

6 Cottage Hill Rd Government Blvd. Girby Rd. Demetropolis Rd. Halls Mill Rd. Figure 2: Horizontal Profile of Spring Creek Source: City of Mobile GeoDatabase, 2007 the pond and then increased as the creek approached Cottage Hill Rd. The elevation again began to decrease on the other side of the road. A very drastic change in the graph occurred towards the mouth of the creek where there were four simultaneous peaks in elevation. This is where Spring Figure 3: Spring Creek at Cottage Hill Rd. Source: City of Mobile GeoDatabase 2007 Creek crosses four major roads, Girby Rd., Government Blvd, Demetropolis Rd., and

7 Halls Mill Rd. At the corner of Government and Demetropolis is an elevated shopping center which also leads to differences in elevation, (Figure 4). With respect to Spring Creek, my research concluded that any morphologic changes were a result of manmade changes to the landscape, through the construction of ponds, shopping centers, parking lots and major roads. Figure 4: Spring Creek at Government Blvd. Source: City of Mobile GeoDatabase 2007 When examining the horizontal profile of Moore Creek, (Figure 5), and making field observations, the conclusion was made that the changes in stream morphology were also due to human activity. The first major change in elevation was where Moore Creek ran underneath Government Blvd. On the south side of Government Blvd. There were a series of drop offs causing the first significant drop in elevation, (Figures 6 & 7). On the north side of Governemnt Blvd., I noticed some sedimentation buildup upstream. The elevation peaked at Government and decreased rapidly due to the drop offs. The other drop in elevation was due to a manmade dam where Moore Creek meets the Montlimar Canal at Halls Mill Rd, Figure 8. There was also some sedimentation buildup

8 upstream from the dam. I did not find any other changes in morphology. Government Blvd. and series of manmade drop offs Moore Creek meets Montlimar Canal at Halls Mill Rd. Figure 5: Horizontal Profile of Moore Creek Source: City of Mobile GeoDatabase 2007 Figure 6: Moore Creek Government Blvd. Figure 7: Moore Creek Government Blvd. My research concluded that changes in Moore Creek were a result of manmade changes to the morphology through the construction of roads, small dropoffs, and a small dam.

9 Figure 8: Dam where Moore Creek meets the Montlimar Canal. Discussion and Conclusions: An emerging interest in environmental quality highlights the importance in determining what the natural and human impacts are on streams and rivers. In the absence of data such as water flow rates, sediment deposits, or other hydrologic parameters, analyzing and monitoring stream morphology through horizontal profiling proves to be very useful and beneficial. With natural impact and human causes such as urbanization and construction, runoff and sediment loads increase, causing geomorphic changes in size and form. In the Spring Creek profile, noticing 4 peaks in elevation due to the presence of roads is a possible indicator that sedimentation might occur between these peaks of elevation. It is extremely important to understand and predict these channel changes since they are often associated with flooding, sedimentation, water quality, and even fish habitat problems (Leopold, 1915). There are so many ways and techniques used to study the physical character of stream channels. Hydrologic parameters such as channel flow direction, flow rates, deposition of sediments, slope, roughness, bedmaterial size, cross sectional shape, erosion/runoff and an understanding of stream

10 morphology, can help identify environmental changes of many sorts (Harrelson 1994). Due to certain technical and timing issues though, geomorphologic analysis on large and complex study areas has been prohibitive. With the advent of Geographic Information Systems, watershed and hydrologic modeling can now be carried out rapidly and accurately. With the completed research, I anticipate that the Dog River Clearwater Revival will gain valuable information on the causes for change in stream channel morphology through the horizontal profiling. It would be in my interests to further my research and to conduct a more complex study on river geomorphology through the use of GIS and field observations in the Dog River Watershed. References Cited: ADEM. Alabama Department of Environmental Management A Survey of the Dog River Watershed Mobile. Chang, H.H Fluvial processes in river engineering. New York: John Wiley & Sons. City of Mobile GeoDatabase, GeoDatabase, no date (20 March 2007) Goodrich, David C., Guertin, Phillip D., and Miller, Scott N Investigating Stream Channel Morphology Using a Geographic Information System. (13 March 2007) Harrelson, C.C., C. Rawlins, and J. Potyondy, Stream Channel Reference Sites: An Illustrated Guide to Field Techniques. USDA Forest Service Rocky Mountain Forest and Range Experiment Station General Technical Report RM-245, 67 p. Leopold, Luna B., Miller, John P., Wolman, Gordon M Fluvial processes in geomorphology. San Francisco, W. H. Freeman, 1964 Russell, Richard J River and delta morphology. Baton Rouge, Louisiana State University Press, Schumm, S. A The fluvial system. New York: John Wiley & Sons.