Identifying the natural flow regime and the relationship with riparian vegetation for two contrasting western Australian rivers

The natural flow regime and the relationship between flows and riparian veg etation are described for sites on both the Blackwood River in south-wester n Australia and the Ord River in north-western Australia. Analysis of long- term flow data showed the historic mean monthly river discharge for the Bla ckwood River is strongly seasonal and highly predictable with generally low variability each month. The Ord River showed a strong seasonality of flows with about 92% of the (total) yearly flow occurring between December and M arch. Flow variability was very high (e.g, coefficient of variation > 100% for all months) but highly predictable, with this mostly attributed to low but constant dry-season flows. Water depth, duration of flood events and th e number of flood events per year show a significant correlation with aspec ts of the riparian vegetation within experimental vegetation plots. Results highlight the strong relationship between floristics, life form structure and population dynamics with stream hydrology. On the Blackwood River, spec ies richness and cover of shrubs reduced with increased duration and freque ncy of flooding, while cover of exotic species and annual herbs increased w ith increased flooding. Germination of tree seedlings was not influenced by flood regime but size class of tree species increased with flooding freque ncy. On the Ord River, species richness was not influenced by flooding regi me. However, cover of perennial grasses increased with flooding frequency w hilst cover of shrubs decreased. There was no relationship between flooding and seedling establishment whilst tree size class decreased with increased flooding. The methods described here can be used to compare the response o f different components of the riparian vegetation to different fluvial regi mes (e.g. because of impoundment and abstraction). This technique can be ex panded for the management of riparian zones and planning rehabilitation pro grammes. It may also be useful for improving the ecological knowledge base for setting environmental flows in regulated systems Copyright (C) 2001 Joh n Wiley & Sons, Ltd.