Changes in the Berg River Basin over time

[摘要] ENGLISH ABSTRACT: The nature of river ecosystems is influenced by the history of activities in their basins. This dissertation investigated historic changes in the Berg River Basin and their influence on river ecosystem structure. The central assumption was that all activities in a river basin landscape contribute either directly or indirectly to the condition (physical, biological, chemical) of rivers that run through them. It was first necessary to establish what changes had taken place in the river basin over time and this was done in different ways at different spatial scales. Changes in land-use were collated and mapped across the basin since these were considered to influence the river's flow regime and river channel structure. Predictions were made about how changes in flow and river channel habitat would influence the distribution and abundance of aquatic macroinvertebrates.A history of land-use changes over the Berg River Basin was explored between four periods, 1955-1965, 1976-1985, 1996-2005 and 2006-2015. The bulk of the dryland crop production was in the lower foothills and lowlands while the upper foothills comprised orchards, vineyards and forestry. From 1955-2015 the extent of agricultural land in the basin declined by half as dryland crops were changed to orchards and vineyards and large tracts of land were left fallow. Over the same period the area under forest declined by 73% and urban areas doubled in size as did the number of farm dams in response to the increased need for irrigation to supply the more water hungry crops. The effects of the changes in land-use, the increase in farm dams and the construction of large dams on the river's flow regime was investigated next.Changes in flow were explored at four river gauges along the length of the Berg River up- and down-stream of the two main in channel dams; the Berg River Dam in the Upper Foothills and Misverstand Dam in the Lowlands. In general the changes were more marked at the downstream gauges and the trends were towards increased dry season flows and slightly decreased wet season flows due to release of water from, and capturing of floods by the in-channel dams to meet irrigation demand in the dry season. Flow pattern from early records was better correlated with rainfall than that from the recent record indicating that flow changes were likely to be attributable to anthropogenic effects such as land-use and water resource developments. Both land-use and water resource developments were predicted to have consequences on river channel shape and habitat that was investigated next.Changes in river channel shape, the extent and composition of the floodplain and riparian area was mapped from aerial photographs at five sites along the Berg River and at five adjacent tributaries. Each site responded differently, which was not unexpected, and reductions in the extent of the channel and riparian area were more severe along the Berg River main stem when compared to the tributaries. Along tributaries no floodplains were discernible at the scale measured, however a decreased in extent over time along the main river except downstream of the Berg River Dam where the floodplain area had increased due to the previously braided channel of 1938 changing to a single thread channel with floodplain and a greater area of sandbanks. Changes in river habitat, such as these, were predicted toeffect change in the abundance and community structure of aquatic macroinvertebrates, which was investigated next.The abundance of aquatic macroinvertebrates was studied from the 1950s to 2015 and showed a reduction in simulids and baetids with an increase in the abundance of chironomids, indicating a decline in water quality. Changes in other groups indicated a decline in quality of habitat, for instance a loss of plecopterans that prefer clean gravel beds being replaced by caenids that prefer a sandy channel bottom. In 2015 there were also more groups of invertebrates that are associated with slow-flowing areas and marginal vegetation, which was presumed to have occurred in response to the clearing of woody alien trees from the river banks and the subsequent proliferation of aquatic and marginal plants along the water's edge.Data collected for land-use, hydrology, channel and riparian changes, macroinvertebrates were synthesized using BEST (BIOENV and BVSTEP) multivariate statistics in PRIMER to search for high rank correlations between environmental and biological variables. When the environmental variables were tested against the biological variables showed that changes in macroinvertebrates were strongly related to area of plantations, area of undeveloped land, the extent of braiding, maximum 5-day average discharge in the wet season and the daily average volume in the dry season. Environmental variables were most influenced/driven by location (separated into sub-basins) while time was the driving factor for the macroinvertebrates data.

[发布机构] Stellenbosch University