Spring 2018
Exploring Channel and Topographic Characteristics Related to Tectonic Deformation in the Ruhuhu River Watershed, Tanzania
Introduction
Lake Malawi exhibited an open hydrologic system that was present even during dry, shallow periods prior to 800ka. Following the lake level transition around 700ka, the lake deepened and became more hydrologically closed with the exception of a single outlet, via the Shire River in the southernmost region. The most feasible candidate for an earlier outlet is the current influent and rift-antecedent Ruhuhu River (Ivory et al., 2016). It was hypothesized (Ivory et al., 2016) that an uplift occurred 700-800ka and altered the direction of flow in the river. For a given tectonic uplift, when one side of the tectonic divide rises, the other side is lowered which reduces the relative base level. After the uplift surfaced, the lake deepened until it reached the modern outlet in the Shire River, where it presently drains through the Zambezi River to the Indian Ocean.
Demonstrated by evidence found in the aquatic evolution of Lake Malawi, the present day Ruhuhu River inlet once served as the outlet and connected the rift lake to the Indian ocean (Ivory et al., 2016). By performing analyses on the Ruhuhu River watershed, we can determine if there are tectonic controls impacting the river’s flow behavior. Digital elevation models (DEM) provided by satellite imagery data will be utilized to assess the fluvial patterns and landform characteristics for evidence of tectonic deformation to test the hypothesis that a structural uplift in the Ruhuhu watershed was responsible for the change in Lake Malawi’s hydrology and biodiversity.
Methods
DEM map illustrating the northeast area of Lake Malawi overlain by the primary Ruhuhu River drainage network of HSO 5-8. Red-shaded areas represent low elevation and green-shaded areas represent higher relative elevation.
Digital elevation data studied in this project was obtained from the NASA Shuttle Radar Topographic Mission (SRTM) database that is distributed by USGS. The highest resolution data for eastern Africa has a 3 arc-second resolution (approx. 90m) and is formatted in 5 deg x 5 deg tiles. The files are georeferenced to the WGS84 datum.
All the data that was gathered for this project was processed and extracted using the terrain analysis and hydrologic modeling capabilities provided by the RiverTools software (version 4.0.2) by Rivix, LLC.
To test the hypothesis that the direction of flow in the Ruhuhu River reversed in response to a tectonic uplift, this study investigates several morphometric parameters to describe the watershed. The morphometric parameters utilized in this study include Asymmetry Factor, Elongation Ratio, Drainage Density, Hypsometric Integral, Shape Factor, Sinuosity, and Stream-Length Gradient Index. These calculations were made for 9 other subbasins identified in 4 zones of high drainage density on the north and eastern margins of Lake Malawi and also for the Ruhuhu drainage network and its 5 independent subwatersheds.
Morphometric parameters
Data
DEM map showing the location of zones and large basins evaluated for the Lake Malawi watershed.
Zones surrounding Lake Malawi were defined to split the large DEM file into smaller, more manageable sizes. For each of the 8 zones defined, the largest HSO streams that had outlets at the lake margin were designated as a basin. Channel profiles were extracted for the largest order reach of the stream. Relief along this profile, profile length, slope, and sinuosity were all measured for these main drainage lines. Additionally, if the stream had one or two large tributaries of one HS order less than the main channel, the criterion was also evaluated for them. When evaluating the drainage network that supplies Lake Malawi, there were no significant basins contributing to the lake on the eastern perimeter except LMZ8, the Ruhuhu River watershed. That is why there are no watershed characteristics described for zones LMZ5-7. LMZ4 basin characteristics are listed as CP12 because there are two large order streams that make up a single basin in LMZ4. The Ruhuhu Watershed, LMZ8, is the only basin that is further divided into subbasins. Watershed analyses was performed on the entire basin (LMZ8Ruhuhu) draining to the main HSO 8 - Ruhuhu R. drainage line and for the 5 tributaries of HSO 6 or 7, LMZ8_CP1-5.
Results
The area was calculated south of the east-west trending channels of the main Ruhuhu River and its tributary referenced as LMZ8_CP3 to determine the asymmetry factor. The area of the Ruhuhu watershed south of this line was approximately 6064 sq. km. Ruhuhu River drainage was shifted towards the downstream southern side of the main drainage line.
Asymmetry Factor (Af). Given by area south of main drainage line in relation to the total basin area. Black arrows point in the direction of basin tilt.
In the basin relief vs. CDF HI plot, Z8CP1 and Z8CP2 basins have high relief values and high CDF HI values. This relationship is characteristic of less mature networks. Z8CP3 and Z8CP4 basins have low relief and CDF HI, indicating these basins are more mature and stabilized. Z8_CP5 and the whole Ruhuhu R. watershed have high basin reliefs and low CDF HI values. The graph illustrating Basin Relief against the elevation-relief ratio method for HIS has a more obvious trend line for the two basin zone categories compared to the basin relief vs. CDF approach to HI.
The slope vs. sinuosity plot reveals that the Ruhuhu subbasins and other basin zones have opposite relationships. As slope increases in the main channels of their respective Ruhuhu subbasins (Z8CP1-5), sinuosity decreases. However, for the HSO 8 Ruhuhu River, there is a very low gradient accompanied by a very high sinuosity. Basin relief plotted against sinuosity demonstrated that the subbasins of the LMZ8_Ruhuhu basin categorically have higher sinuosity compared to the LMZ1-4 basins despite having large basin relief.
Any SL index computed for an entire channel length will simply be equal to the channel’s change in elevation. SL was only computed for 4 segments within the Z8_Ruhuhu basin. Each of the 4 segments proved to have significantly high SL values, emphasizing how sensitive drainage networks and channel profiles are to knick points.
Demonstrated by the elongation vs. drainage density plot, the Z8Ruhuhu subbasins skew slightly more elongated than the other zones. Z8CP1 and Z8CP3 are significantly more elongate than the rest. The Ruhuhu R. watershed has lower values of drainage density compared to other basins, illustrated in the shape factor and drainage density plot.
Discussion
The values reported for the basin relief vs. CDF HI plot suggests the uplift event most likely occurred on the north/northwest area of the Ruhuhu watershed and tilted the basin south of the main channel (Ruhuhu: HSO 8). The highest values for SL index of the four channel reaches tested occurred in the main channels of the Z8CP1 and Z8CP2 basins, 797 and 880 respectively. The latter is especially important because it occurs just upstream of the CP2 and Ruhuhu R. confluence, which is also immediately downstream of the Ruhuhu’s second water gap. Because Z8CP1 exhibits high basin relief, high sinuosity, high CDF HI, a high elongation ration, and low shape factor, the uplift event most likely originated within close range of this basin.
A river will try to maintain its channel slope so when there is an uplift, the downstream side of the uplift will have been raised and have a steeper valley slope. In response, the river channel increases sinuosity to preserve its original channel slope. The area upstream of the uplift will straighten and decrease sinuosity since the overall stream valley slope has decreased. If the valley slope increases in the opposite direction, a normal fault has downthrown the upstream side resulting in downstream straightening and incision (Zamolyi et al., 2010).
Conclusion
Ruhuhu River Watershed.
Various geomorphic parameters describing watershed responses to tectonic uplift were quantitatively measured and spatially evaluated to determine if a tectonic origin is responsible for the topographic and drainage network expression within the Ruhuhu watershed. The data collectively demonstrates that the westward flow of the Ruhuhu River is a fluvial response to a tectonic uplift that constrained the basin perimeter to its present day location. Watershed characteristics for the Ruhuhu basin and its subbasins revealed relationships associated with structural influence. Nine other inlet basins of Lake Malawi were evaluated similarly and plotted with the Ruhuhu watershed values. This allowed me to distinguish how much the Ruhuhu is influenced by certain attributes in comparison to other similar, more stable bedrock rivers in the same geographic setting. Apart from the numerical assessment, the delineated Ruhuhu watershed exhibits some eastward directed tributaries and also heavily incised meanders, observed through Google Earth. The exact location of where the uplift occurred is unknown. However, given the presence of two large water gaps along the main channel of Ruhuhu R., the rate of incision outpaced the local rate of uplift.
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