Distribution and hydraulic significance of large woody debris in a lowland Australian river

The line-intersect technique was used to measure the loading of large woody debris in a 1.8 km reach of the Thomson River, Victoria (catchment area of 3540 km²). A debris census (measuring every item present) was done over 0.775 km of this reach. The transect technique over-estimated the actual loading revealed by the census. The loading of debris 0.01 m in diameter for the total 1.8 km reach was 0.0172 m³ m^–2, which is higher than that measured in many headwater streams in other parts of the world. The volume loading of debris measured from low level aerial photographs was only 4.8% of the value estimated by the line-intersect technique. The line-intersect estimates were biased due to non-random orientation of debris in the stream (causing estimated errors of +8% for volume loading and +16% for surface area loading). It is recommended that to avoid this problem, when using the line-intersect transect technique in lowland rivers, each line should comprise at least two obliquely-angled transects across the channel. The mean item of debris (0.1 m in diameter) had a trunk basal diameter of 0.45 m, a length of 7.4 m, and volume of 0.7 m³. The riparian trees and the in-channel debris were of similar dimensions. The debris tended to be close to the bed and banks and was oriented downstream by the flow at a median angle of 27°. Because of this orientation, most debris had a small projected cross-sectional area, with the median value being only 1 m². Thus, the blockage ratio (proportion of projected area of debris to channel cross-sectional area) was also low, ranging from 0.0002 to 0.1, with a median value of 0.004. The average item of debris, which occupied only 0.4% of the cross-section, would have minimal influence on banktop flow hydraulics, but the largest items, which occupied around 10%, could be significant. Judicious re-introduction of debris into previously cleared rivers is unlikely to result in a large loss of conveyance, or a detectable increase in flooding frequency.     

Assessment of the ecological health of heavily utilized,large lowland rivers: example of the lower Yellow River,China

We describe an approach to assessing biological health in a heavily utilized, large lowland river setting, using the example of the lower Yellow River, China. In this study, the river was divided into four reaches, and a field survey of fish, benthic macroinvertebrates, and riparian plants was undertaken. Data from historical records were used for reference. The scoring system used weighting to adjust for differences in the importance of the chosen indicators and the relative efficiency of the sampling effort. The results indicated that the biological health of the lower Yellow River is distant from historical reference conditions, with its condition generally declining in the downstream direction. This result is consistent with the river’s history of impairment of hydrology, water quality, wetland area and character, and physical form. We conclude that the reference river state based on historical conditions is unlikely to equate to the best attainable river health because certain changes have occurred in the river that may prohibit a return to the previous state of ecological health. On the basis of the results of this assessment as well as a review of the literature, we propose field data collection methodologies and indicators that can be applied in future assessments.     

The use of turbidimeters in suspended sediment research

Accurate characterisation of the trend in suspended sediment concentration in streams throughout runoff events requires a sampling interval much shorter than the one hour typically used. For logistical reasons this is not usually feasible, so continuously recording turbidimeters offer a potential alternative. The variety of correlations and relationships between sediment concentration and turbidity reported in the literature is not surprising in view of the many confounding effects present in natural stream systems. Data from five sites in a small forested catchment indicated variability in the turbidity-suspended sediment concentration relationship, but improvements were obtained by considering the effects of very fine sediment and background water colour. Throughout runoff events variations in sediment properties, organic acids concentration and turbulence can occur. It is concluded that a good linear relationship between turbidity and suspended sediment concentration should not necessarily be expected and any observed hysteresis could actually help explain erosion and transport processes. The detail and continuity of data generated by turbidimeters provides the opportunity for greater understanding of storm sediment dynamics, however, calibration requires careful consideration of site characteristics.     

A freshwater ecoregion delineation approach based on freshwater macroinvertebrate community features and spatial environmental data in Taizi River Basin, northeastern China

The ecoregion is currently widely used as the basic geospatial unit in freshwater biodiversity conservation. The popularly used delineation is usually based on the assumption that attributes of aquatic ecosystems are influenced by landscape-scale environmental variables. However, few ecoregion delineations attempt to establish the local validity of this assumption prior to delineation, and few studies check the correspondence of the derived ecoregion boundaries with the distributions of attributes of aquatic biota. In this study, we established an approach to overcome these shortcomings. The notable features of the approach are: (1) the delineation variables were filtered through a series of analytical steps to select those that best represented the aquatic community traits, and which avoided redundancy in the data; (2) the method was quantitative and repeatable; and (3) the derived ecoregion boundaries were checked for consistency with the spatial attributes of aquatic biota. The approach was applied in the Taizi River Basin, northeast China. The procedure proposed here filtered out altitude and annual precipitation as the best variables to include in the freshwater ecoregion delineation. Then, using the quantitative ISODATA classification method, the basin was classified into three ecoregions. A test of accuracy indicated that freshwater ecoregions matched well with the spatial distribution pattern of macroinvertebrate community attributes. Statistical analysis showed that natural geographical attributes and river attributes were different in the three ecoregions, and indices representing macroinvertebrate community attributes are significantly different as a whole among the three ecoregions. The case study proved this approach effective on ecoregion delineation.