Changing land use, changing river channels: A study of six watersheds in the Venezuelan Andes.
The proposed study will analyze the relationship between change in watershed land use and river channel shape over time. Aerial photographs of six watersheds in the foothills of the Venezuelan Andes will be analyzed from two time periods, the 1940s and the 1980s to determine river channel width, sinuosity, and position, relative to land use. This study seeks to combine two analytical frameworks by examining multiple riparian landscapes over a forty-year period in time.
Recently, increasing emphasis has been given to the effects of watershed land use on several stream characteristics (Allan, Erickson & Fay 1997). Jacobson and Pugh (1995) found evidence to link changes in the shape and position of a single stream channel to changes in the land use surrounding the stream channel. In a different study, Allan, Erickson, and Fay (1997) found similar correlations between changing land use and other, more specific, physical properties of streams, such as sedimentation and runoff. Both studies used a geographic information system (GIS) to model or predict the effect of a shift in land use on the stream’s attributes.
Although there have been in-depth studies of single watersheds, and some studies of adjacent geographical areas, few studies examine multiple watersheds over time. Several of the existing studies examine single hydrologic variables, such as sediment loading or runoff. However, hardly any studies can be found that examine the river channel as part of the larger watershed landscape. This study seeks to address a series of watersheds at this scale.
The proposed study seeks to answer the following research question: How does land use affect stream channel morphology? To answer to this question, I will analyze a series of streams that stretch across the Southern slopes of the Venezuelan Andes. These watersheds are the subject of on-going stream ecology research at the School of Natural Resources & Environment. Each watershed has its own, unique pattern of land development over the past half century. While each watershed has undergone land-use change, some watersheds have changed more than others.
The analysis of the watersheds will be based on analysis of remote sensing images, primarily black and white panchromatic aerial photographs. Techniques that examine rivers in this manner are called landscape approaches because they capture multiple attributes of the stream and its surrounding landscape. As the technology improves, these techniques are becoming increasingly popular to analyze streams and predict their future behavior (Johnson & Gage 1997). This study will utilize landscape approaches to study riparian landscapes over time.
This study has major implications for future land-use planning. Knowledge of the effects of land-use change on a river can be used to plan for future land development and future riparian integrity. Once the intertemporal relationship between rivers and their surrounding land use is better quantified, planners can take trends of the past into account when planning for the future.
Simpson, et.al., (1994) claim, “Although landscape structure and dynamics have now been quantified in a number of specific settings, most have examined either a single landscape over time or multiple landscapes at a single point in time. As a result, the collective body of knowledge is still inadequate for a general model of rural land-cover change.”
In fact, literature that covers the entire scope of the proposed project has not been found. However, an extensive amount of literature can be drawn upon to address individual pieces of the proposed project.
Simpson, et.al., (1994) analyzed the change of two adjacent landscapes over time. Their landscapes differed in topography as well as pattern of settlement and development. They found that the pattern of development over the forty-eight year study period differed between the two study areas. This difference related to both the underlying geology of the each area and the economic conditions of its communities. The study discusses the importance of socioeconomic development on the change in the landscape as a whole.
Allan, Erickson, and Fay (1997) analyzed development of one river and its watershed over a period of time. They found that “Human alteration of the land affects river ecosystems through multiple processes that likely operate at different spatial scales.” Much of their discussion focused either on socioeconomic development of land, particularly the transition of agriculture to subdivisions, and more specific hydrologic and biological changes of the river. Their models showed an increase in both runoff volume and sediment yield in the flow of river water with increasing agriculture and urbanization of the watershed. However, these discussions were not compiled and analyzed as a group to discuss a change in the river landscape.
In their 1995 United States Geological Survey study, Jacobson and Pugh (1995) analyzed one river over a fifty-year period from aerial photographs. They found that the river had moved over time, and that land use surrounding the channel has also changed over time. This watershed had much of its development in the valley, and Jacobson and Pugh concluded that a change in land use in the river valley impacted the river channel itself.
The methods of this project have two components, individual watershed analysis and comparative watershed analysis. The first stage entails an analysis of the stream channel and its surrounding land for each watershed. The development of a protocol for individual watershed analysis is currently underway. After individual watershed analyses, the collected data will be compared across watersheds, to examine regional trends.
Aerial photographs covering upstream and downstream sites on each of the six proposed rivers have been obtained in stereoscopic pairs. Two sets of photos exist for each of the sites – one from the mid-1940s, the other from the early 1980s. Additionally, two Landsat images covering the entire region have been obtained. The aerial photographs will all be registered to points of known location on the satellite images. These photographs will be analyzed in an image processing program (eg. ERDAS Imagine 8.3) and compared in a GIS (eg. ArcView 3.1).
For individual watersheds, image processing software will be used to warp the aerial photographs of different time periods to the same scale. Once at the same scale, the photos can be overlaid to compare river channel position across time. The image processing software will also be used to obtain channel width, sinuosity, and percent land-use measurements for each time period.
The measures of change in river channel width, sinuosity, and percent land use will be compared across all of the watersheds in the study. Relationships will be drawn between change in river width and sinuosity and change in percent land use over the forty year period.
I expect to find that the river channels that have experienced the most intensive urban and/or agricultural land development will have changed the most. I expect these channels to have widened the most and actually have changed position over the time period of approximately forty years. Findings that land use has a direct correlation to river channel shape can be applied to the management and planning of watersheds.
Allan, J.D., Erickson, D.L., Fay, J. (1997) The influence of catchment land use on stream integrity across multiple spatial scales. Freshwater Biology, 37, 149-161.
Jacobson, R.B., Pugh, A.L. (1995) Riparian-vegetation controls on the spatial pattern of stream-channel instability, Little Piney Creek, Missouri. U.S. Geological Survey Water-Supply Paper 2494.
Johnson, L.B., Gage, S.H. (1997) Landscape approaches to the analysis of aquatic ecosystems. Freshwater Biology, 37, 113-132.
Simpson, J.W., Boerner, R.E.J., DeMers, M.N.,
Berns, L.A., Artigas, F.J., Silva, A. (1994) Forty- eight years of
landscape change on two contiguous Ohio landscapes. Landscape
Ecology, 9, 261-270.