A comparison of deltaic structure in forested and deforested watersheds using highresolution bathymetric surveys
Student: Willie Guerra
Mentor: Kiram Lezzar
Introduction
Steeply sloping deltaic systems present in the rift environment of East African lakes were studied
at a large scale in Lake Malawi (Johnson et. al 1995), with detailed bathymetric maps created for major
river deltas. This study failed to show the impact of land use on the sedimentary environment in the lake.
Two smaller deltas systems on the eastern coast of Lake Tanganyika were selected for Nyanza Project 2007
to investigate the same relationship. They display great contrast in terms of human impact, one intensively
deforested, and the other protected, and intensively afforested under the strict guidelines of the national
park service. It was thought that the differences in land use would give rise to a different deltaic structure.
Methods
The Nyanza Project 2007 used the R/V Echo to build a dense grid of bathymetric data. The overall
strategy was to record the water depth every three seconds while maintaining a constant speed and constant
direction. These depth transects, as they will be henceforth known, were dependent on the knowledge of
start and end GPS coordinates, as well as coordinates at 1 minute intervals to confirm that the ship was
holding a straight course. If it can be assumed that the ship was aligned straight and at a constant speed
over each one-minute interval, then the coordinates of every three-second depth reading can be deduced by
extrapolation from the start and end coordinates of the one-minute intervals. For the depth readings, the
team used two Sonar Devices – the Raytheon model that the boat is equipped with, in addition to a smaller,
though more stable, Lowrance fish finder. Two handheld GPS units were also required, one for navigation
and a second for confirmation of the ship’s location at one minute intervals. In practice, the task of
recording depths every three seconds while navigating a straight course, confirmed consistently by GPS
coordinates required a minimum of 5 people.
Before arriving at the field site, potential depth transects were identified using topographic maps
to create grids of approximately the same size and shape around each river’s mouth. During each transect
there were five distinct jobs on the boat. The navigator’s duty was selecting routes based on the preliminary
maps and signaling the start and end of each depth transect for the rest of the team. On site, the navigator
attempted to follow the proposed transects using the GPS unit, but due to wind and other boat issues
resigned to following transects in close proximity to the planned route. The GPS recorder would log GPS
coordinates at the start and end of each transect, as well as every minute during the route. Announcing
depth soundings required one person using a stopwatch to tap another person next to the sonar device to
signal the announcement of a depth every three seconds. There was also one person recording the depth
readings directly into an IBook laptop. At Mtanga, a second person wrote the depths into a field notebook
to back up the data, but limitations on the field crew did not allow for this at Kasekera. There were 16
transects completed at Mtanga and 32 completed at Kasekera stream. Kasekera stream was covered more
completely due to a more evenly spaced grid of depth transects. In addition to the points collected on the
boat, the team mapped the shoreline with GPS points. Two people landed on shore and recorded the GPS
coordinates of the water’s edge using the same handheld units from the boat. These people walked the
entire length of the study area, following the water and recording a coordinate at every turn in the shoreline.
For the creation on the bathymetric delta maps, the depth readings were converted to X,Y,Z
coordinates in the form x= latitude (minutes), y= longitude (minutes), z depth (meters). This was
accomplished by extrapolating the latitude and longitude between each one-minute GPS interval. These
X,Y,Z coordinates were plotted in a three dimensional scatter plot, using Sigma Plot version 10.0. After
visually confirming the locations of transects, the data were then smoothed using a median smooth with
60x60 resolution. The smoothed data were also used to generate a contour plot of each delta, in addition to
cross sections on and off delta in each river system.
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Maps generated to show the entire river system, delta, and land surface utilized data from regional
topographic maps. The land area around each river was arranged into grids in the same manner as the water
over the delta. Elevation transects were traced as east-west lines extending onto land from the river’s mouth
to a predetermined line drawn on the map, approximately 300 m from shore at each river mouth. A point
was recorded at every contour line (10m). X,Y,Z values in the same format as the bathymetric grid were
produced from the latitude and longitude coordinates on the map. These points were then used to generate
the necessary number of coordinates for the grid by extrapolation of the elevation values between each
contour. The extrapolated points were plotted in Sigma Plot, with the shoreline from the bathymetric data,
as a three-dimensional scatter plot and smoothed using a median smooth. This created a three-dimensional
model of the land immediately surrounding the river mouths. The map was then added to the plot of the
lake bathymetry by manually removing the points below the water’s surface from the smoothed data, and
adjusting the longitude coordinates to correspond with the hand mapped shoreline. For the maps to fit
more evenly, the water level was dropped by 1.5 m.
The sizes of the watersheds were calculated using scanned copies of the local topographic maps.
With the outline of the watershed manually drawn onto the digital image, the area enclosed was calculated
with the program TPS dig. This program was also used to calculate the size of the individual deltas by
tracing the delta lobe along the 30m depth of the contour plot.
Results
Major differences between the study areas were evidenced by the sizes of the respective
watersheds and the sizes of the deltaic systems. The project had intended to examine the differences
between the forested Kasekera stream and deforested Mtanga, but on close examination of the delta
structure, decided to contrast a combined Kasekera/Mkenke (KMK) system with the Mtanga. Mtanga’s
watershed encompasses an area of roughly 6.76 km2. This is significantly larger than Kasekera’s at 3.32km2
or Mkenke’s at 2.82km2 area. The size of the delta lobe is approximately 0.46km2 at Mtanga, and the total
delta lobe area at KMK is 0.40km2. The ratio of watershed size to delta size is thus 0.068 at Mtanga and
0.065 at KMK. Great differences are visible in the deltas of the two systems, but the large-scale structure
remained similar. Both systems display the alternating canyon/delta platform structure, though in KMK, the
canyons are much more pronounced. The slope of the KMK delta is slightly steeper up to about 40m (the
slopes are comparable after that). It is also more elongated and nearer to shore. The position of the canyons
in relation to the topography of the adjacent shoreline is very well correlated. Canyons seem to match up
very well with areas of steep relief and deltas with low relief areas.
Discussion/Conclusions/Future Study
The relative size of delta systems to watershed size is well matched between the two systems, but
without a larger sample size, it is impossible to gauge the significance. The impact of deforestation on delta
size and structure appears negligible as well, though without a larger sample, this is also inconclusive. The
locations of the deltas do not necessarily match with the locations of the present flowing stream at
Kasekera, implying that the location of the largest stream may have shifted over time. Conversely, the
variation could be a product of local tectonics. A similar project exploring a large number of small delta
systems could have considerable value for interpreting the lake environment for a variety of purposes,
including the impacts of human land use. This study, though insightful for adding perspective to grab
sample locations, is not sufficient for interpreting the impacts of deforestation on deltaic structure.
Acknowledgements
Thanks to Kiram Lezzar for continuing to push the project in new directions and give
indispensable insight throughout the project. On the boat, my first mate and navigator Marc Mayes, head
depth caller Jon Husson and GPS leader Anthony Romano, and Chief Tapper Sara Sipahioglu, and of
course Captain Chale for impeccable navigational ability (and a knowledge of water depth occasionally
better than sonar). Thanks also to Chata and Mupape for keeping the expedition moving forward by
repeatedly repairing the sonar, propeller, battery (and so on). Jon Todd came through when needed on the
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boat calling depths and on land talking through the sometimes-puzzling delta structure. Thanks also to the
people who filled in when a little extra help was needed, including Dustin Wing, Caitlin Keating, and
Morgan Helfrich. A final thanks to TAFIRI for allowing the project to use their facilities the length of the
program. This project was supported by NSF Grant: ATM-0223920 and DBI-0608774.
References
Johnson, T.C., Wells, J.D,, Scholz, C.A. 1995. Deltaic Sedimentation in a modern rift lake. GSA Bulletin:
vol. 107, no.7, p. 812-829.
Figure 2:
Combined Kasekera/Mkenke waterdshed
Size: 6.14 km2
Kasekera Watershed: 3.32 km2
Mkenke Watershed: 2.82 km2
Figure 1: Mtanga Watershed
Size: 6.76km2
Kasekera
Figure 3: Depth Transects Mtanga
20 total
Mkenke
Figure 3: Depth Transects Kasekera,
Mkenke (KMK)
38 total
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Kasekera
Mkenke
Figure 6: Kasekera System. Topography
extends ~300m from shore
Figure 5: Mtanga System. Topography
extends ~300m from shore
Figure 7: Kasekera Contour Plot
Grab samples included
Figure 8: Mtanga Contour Plot.
Grab Samples included
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Figure 9: Mtanga delta system, with grab
samples included.
Figure 10: Kasekera delta system, with
grab samples included.
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Figure 11: Mtanga delta cross section, with
grab samples included.
Figure 12: Mtanga off delta cross section,
with grab samples included.
Figure 13: Kasekera on delta cross section,
with grab samples included.
Figure 14: Kasekera off delta cross section,
with grab samples .included.
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