River and landslide dynamics on the western Tanganyika rift border, Uvira, D.R. Congo: diachronic observations and a GIS inventory of traces of extreme geomorphologic activity

Uvira occupies a series of narrow alluvial fans squeezed between the NW corner of Lake Tanganyika (±710 m asl) and the W-shoulder of the Tanganyika rift, the Itombwe–Mitumba Plateau (±3,000 m asl). In 50 years, the fans progressed into the lake over distances up to some hundreds of metres. This happened during a few catastrophic flash floods issued from the torrents which cascade from the rift shoulder with a mean longitudinal gradient of 0.2 m m −1 . The last event in 2002 led to the destruction of parts of the town and to some 50 casualties. Landslides occurred in the hills. On the base of stereoscopic interpretation of aerial photographs from 1959, complemented with data from 2000 ETM and 2004 IKONOS imagery, a geographical inventory has been made of strongly incising (10 −1 to 0 m in 43–45 years) river sections, of all types of landslides and of all tectonic structures, visible in the rugged hinterland of the fans. Traces of active N–S as well as E–W trending faults are present. Some of these faults and some surfaces, interpreted as degraded fault facets dip at angles of 40° or less and are probably remnants of formerly active lystric extension faults, originally at a depth of some 2 km, but now at the surface as a result of posterior uplift and erosion. Sixty landslides could be identified. Six slides fall far below the topographic threshold envelope, where the slope at the incision head is expressed as a function of drained surface. Therefore, they are considered to be seismic in origin. Most of the other landslides are located along strongly incising river sections. Temporary landslide barriers contribute to irregular river hydrographs. It is concluded that Uvira is threatened by landsliding, potentially massive (>18 × 10 6 m 3 debris), in the case of heavy seismicity. It is further discussed that the regularisation of the river regime depends on soil and water conservation strategies, to be developed in the headwaters of the torrents Kavimvira, Mulongwe and Kalimabenge. Copyright Springer Science+Business Media B.V. 2010