A river health assessment of selected South-Western Cape Rivers : index of habitat integrity, water quality and the influence of surrounding land use

[摘要] ENGLISH ABSTRACT: The River Health Programme (RHP) is an assessment tool for monitoring the ecological state ofrivers to ensure that they remain fit for use by present and future generations. This study, formingpart of a RHP assessment conducted on the south-western Cape Hout Bay, Lourens and PalmietRivers, has the aim to (1) zone the rivers for representative site selection, (2) assess their habitatintegrity (HI), (3) determine the influence of land use on riverine HI and (4) assess the river waterquality at the time of the RHP assessments.(1) The desktop geomorphological zonation method used in RHP assessments has not beensufficiently previously tested on short rivers draining the Western Cape Mountains. TheLowland River Zone of the rivers studied, as well as the Hout Bay River's Upper Foothill Zone,were found to have steeper gradients than expected, probably due to these rivers being shorterand consequently steeper than any on which the method was previously tested. The notion ofone gradient river classification system being applicable throughout South Africa, with itsdiverse geology and climate, is unlikely. Rather a classification system modified for variousphysiographic features regions or by a factor based on river length is more realistic.(2) Although there is a general longitudinal decrease in HI downstream along the Hout Bay andLourens Rivers, coinciding with increased anthropogenic activities, HI improves in the PalmietRiver's lower reaches through the Kogelberg Nature Reserve. Surrounding land use thus seemsto be a major determinant of HI. Although the Index of Habitat Integrity (IHI) used appears toachieve its aim, it was found to be subjective. Categorisation of the IHI scoring is suggested.(3) The amount of natural versus disturbed land use occurring upstream of a site at a regional andlocal scale, is a good predictor of riverine HI. Regional alien forestry and local urbanisationhave significantly strong negative effects on instream (r2 = -0.80, r2 = 0.80, p<0.05) and riparian(r2 = -0.81, r2 = -0.83, p<0.05) HI. Different land use types therefore appear to affect riverine HIat differing scales and thus managers must not only think on a local but also a catchment scale.(4) In the Hout Bay River, a filtering system (e.g. wetland) appears to improve the water qualitybetween the middle and lower reaches. Along the Lourens River, high total dissolved salts,conductivity and inorganic nitrogen concentrations in the middle reaches are cause for concern.Along the Palmiet River there appeared to be insufficient oxygen to support most aquatic lifeforms at Grabouw. Impoundments in the middle reaches act as sinks for nutrients and salts, butthe Huis and Krom tributaries downstream then appear to degrade the water quality of thePalmiet River's lower reaches within the Kogelberg Nature Reserve.Together with the results of simultaneous biotic assessments, these results should be used todevelop management actions to improve the ecological health of these rivers. The results have beenused in a State-of-Rivers Report for the south-western Cape.

[发布机构] Stellenbosch University