Report prepared by Steve & Geraldine Janicke
SUMMARY
This report outlines the results of a detailed survey of the environmental condition of 43 kilometres of the Middle Pallinup River floodway. The Pallinup is a highly degraded river and the report rests its assertions and conclusions relative to this. The survey repeated an assessment that was made in the spring of 2001 and added mapping of the environmental values as they were in 2017. The aim of the 2001 riparian condition assessment was to produce a condition map for the river, but lack of resources prevented the completion of this. However, the core data was a comprehensive series of 155 geo-referenced photo points that were established to provide a means to determine medium to long term changes.
Anecdotal evidence suggested that the 1955 flood, following close on the heels of widespread land clearing, dramatically degraded the river’s riparian environment and established its current form. The Middle Pallinup Floodway Condition Survey offered evidence that the Middle Pallinup River had moved past the post-clearing high risk phase and that the changes in channel form due to the 1955 flood still define the floodway. The current form of the floodway is relatively stable with respect to floods of the order of magnitude of the 1982 and 2017 events. Nevertheless, the fragmented distribution of trees, predominantly sheoak and the replacement of understory shrubs and sedges with annual grasses, continues to weaken the floodway and erosion features indicate this. If the frequency of the rare big floods increases these weaknesses can be exploited to aggravate erosion and introduce more sediment into the system. This justifies re-vegetation projects to increase the density of vegetation along the main river floodway and along the tributaries.
The primary aim of the 2017 Floodway Condition Survey was to determine whether rehabilitation efforts in the catchment over the past two decades have made a difference to the environmental and water condition of the river. A secondary aim was to repeat and complete 2001 riparian condition assessment.
Water condition is a broader concept than basic water chemistry measures. It is a product of a range of riparian attributes such as the structure of the river channel itself, in-stream and fringing vegetation, water depth, shading, habitat types and the diversity of the aquatic and terrestrial biota that rely on the water. For this reason, data describing these features also describes water condition, particularly for the plants and animals that depend on the river for survival.
This report describes the character of the river and catchment and the method used to qualify and quantify the changes that have taken place over a sixteen-year period. These changes reflect both the seasonal and landscape pressures acting on the river and the impacts of extreme flood events. Fortuitously, a major flood in February 2017, while perhaps obscuring the impacts of the general pressures, also provided a means to examine the resilience of the river to a flood comparable with the 1955 summer flood.
The key findings
What stands out in the above data is that basic channel structure has remained much the same at the majority of sites over the 16 years period, in spite of various moderate floods and the recent major flood.
Bank, bed and terrace erosion increased only minimally over the sixteen period since 2001.
Sediment deposition increased at 37.5% of the photo point sites with fresh sediment is still being mobilized into the river.
Tributaries are continuing to contribute excessive amounts of fresh sediment into the main river floodway, according to their size, however reworking of existing sediment deposits appeared to be the dominant source of mobile sediment.
Aerial image interpretation of the amount of riparian vegetation along 262 kilometres of the main trunks of tributaries to the Middle Pallinup indicated that approximately 52% of the length has a reasonable cover from remnant vegetation and re-vegetation. Riparian rehabilitation along the tributaries of the Pallinup can in principle be considered to be having a positive effect on the river in proportion to those figures.
Both the Warperup and Jackitup creeks appeared to have substantially increased the discharge in the Middle Pallinup River during the February 2017 flood as evidenced by large erosion scours, large sand deposits and tree damage near the confluences.
By far the most common tree growing along the Pallinup River channel and sandy terraces was swamp sheoak open woodland with soursob and annual grasses understory. Upslope was generally eucalypt open woodlands dominated by York gum, but with patches of yates or flooded gums. Jam wattle and perennial veldt grass dominated the understory.
Eye witness accounts of the effects of the 1955 and 1982 floods speak of a complete tree loss along the floodway. However, numerous old and large sheoaks along the survey reaches suggests this scale of impact may have been more prevalent in the lower Pallinup rather than the surveyed middle reaches. The perceived magnitude of the large floods will depend on which part of the catchment the observer is in.
Flooded gums showed the greatest decline in health and rising saline groundwater levels likely to be the main cause.
There was natural regeneration of swamp sheoak and jam wattles along the floodway but there was negligible natural regeneration of eucalypts.
Samphire response was variable with an increase at 16% of photo point sites and a decrease at 39% of the photo point sites. The Foreshore survey revealed that samphires have a limited ability to armour the channel and terraces from erosion.
The dominant weeds observed along the lower floodway slopes of the surveyed Middle Pallinup reaches were soursob and annual grasses while veldt grass dominated the upper slopes.
Weeds of concern were african boxthorn, prickly pear, bridal creeper and doublegee.
The Pen-Scott foreshore condition assessment graded the foreshore as predominantly C grade, erosion prone.
Fencing for stock exclusion to the river floodway enables germinating trees and shrubs to become established. The left bank (northern side) had 76% protection from stock and the right bank (southern side) had 57% protection.
Rehabilitation planting along the Pallinup main floodway has incrementally improved the biodiversity and habitat character of the river but its contribution to improving the main channel water condition and moderating erosion processes has been negligible (in proportion to the amount of revegetation works along the surveyed reach). However, fencing for stock exclusion appears to have contributed considerably to floodway stability and hence water condition.