A quantitative geomorphic study of the Riberão do Mandaguari, São Paulo, Brazil

Abstract

Fifty-three basins with area less than 9.40 km² were mapped from 1:25,000-scale aerial photographs to provide data on basin morphometry. A significantly larger number of first-order streams exist than are expected according to the regression of stream number on order for each basin. This is attributed to the incorporation of recently formed gullies into the stream network as first-order streams. Most drainage density (Dd) values are in the coarse texture category (Dd<5.0 km/km²). Higher values of Dd correspond to areas of shallow soils. Total stream frequency (F[subscribe t]) and first-order stream frequency (F₁) are closely correlated with Dd (r=+0.86 and r=+0.89, respectively), but third-order stream frequency (F₃) is not (r=+0.70). F[subscribe t] is related to Dd as follows: F[subscribe t]=0.704 (Dd)²̇⁰⁸. Values of ruggedness number (HDd) and basin magnitude indicate that all basins are high-flood potential basins. This imposes serious limitations on the development of the watershed as a whole. Weighted mean percent silt-clay (M) and channel width-depth ratio (F) for 21 cross sections of stream channels were used to define cross section stability. Catchment area above each cross section is related to its stability. As area increases cross sections change from degrading to aggrading to stable conditions. There is a more gradual increase in F with decreasing M than in streams in the mid-western United States. This is attributed to the vigorous vegetation which stabilizes channel banks in the Mandaguari watershed. Recent climatic changes in the region rendered colluvial deposits susceptible to increased erosion under the present subtropical (Cwa) climate. When saturated, the latosols and podzols there are easily eroded owing to their low cohesion (PI=0-15). Recent deforestation and seasonal burning of pastures probably exacerbate this condition, contributing to the erosion of hillsides and the formation of gullies. Large, aggressive gullies (voçorocas) 10-15 m deep and 30-40 m wide occur in association with urban centers. Management of the Riberão do Mandaguari watershed should be based on considerations of (1) threshold conditions for instability of stream channel cross sections and soils and (2) complex responses of the watershed. Recommended actions are reforestation of slopes at and above the point of maximum profile steepness, induced incision of channel filling deposits, and control of runoff from urban centers.