Changing land use, changing river channels: A study of six watersheds in the Venezuelan Andes.
Abstract
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.
Introduction
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.
Literature
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.
Methods
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.
Expected Findings
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.
Works Cited
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.