High-gradient Systems Research Articles

Bioassessment tools in novel habitats: an evaluation of indices and sampling methods in low-gradient streams in California

Biomonitoring programs are often required to assess streams for which assessment tools have not been developed. For example, low-gradient streams (slope<or=1%) comprise 20-30% of stream miles in California and are of particular interest to watershed managers, yet most sampling methods and bioassessment indices in the state were developed in high-gradient systems. This study evaluated the performance of three sampling methods [targeted riffle composite (TRC), reach-wide benthos (RWB), and the margin-center-margin modification of RWB (MCM)] and two indices [the Southern California Index of Biotic Integrity (SCIBI) and the ratio of observed to expected taxa (O/E)] in low-gradient streams in California for application in this habitat type. Performance was evaluated in terms of efficacy (i.e., ability to collect enough individuals for index calculation), comparability (i.e., similarity of assemblages and index scores), sensitivity (i.e., responsiveness to disturbance), and precision (i.e., ability to detect small differences in index scores). The sampling methods varied in the degree to which they targeted macroinvertebrate-rich microhabitats, such as riffles and vegetated margins, which may be naturally scarce in low-gradient streams. The RWB method failed to collect sufficient numbers of individuals (i.e., >or=450) to calculate the SCIBI in 28 of 45 samples and often collected fewer than 100 individuals, suggesting it is inappropriate for low-gradient streams in California; failures for the other methods were less common (TRC, 16 samples; MCM, 11 samples). Within-site precision, measured as the minimum detectable difference (MDD) was poor but similar across methods for the SCIBI (ranging from 19 to 22). However, RWB had the lowest MDD for O/E scores (0.20 versus 0.24 and 0.28 for MCM and TRC, respectively). Mantel correlations showed that assemblages were more similar within sites among methods than within methods among sites, suggesting that the sampling methods were collecting similar assemblages of organisms. Statistically significant disagreements among methods were not detected, although O/E scores were higher for RWB samples than TRC. Index scores suggested impairment at all sites in the study. Although index scores did not respond strongly to several measurements of disturbance in the watershed, percent agriculture showed a significant, negative relationship with O/E scores.

Early evolution of submarine channels offshore Angola revealed by three-dimensional seismic data

Abstract The deep-water subsurface offshore Angola is characterized by many linear, high-gradient submarine channels typically only tens of metres wide and deep. Larger channel systems ( C. 3–5 km wide, &gt;300 m deep) with highly sinuous channels at their bases are also common, although they appear to have evolved from small, linear, high gradient systems. Generally, such small linear channels become enlarged by sediment gravity flows and therefore are rarely preserved except in examples where avulsion occurs. These small linear systems are often associated with relatively continuous levees 1–3 km wide flanking the channel. Results presented here suggest that small, linear channels evolve from erosional lineations on the slope generated by large, infrequent turbidity currents. Results also indicate that linear, high-gradient channels also exhibit the most significant and distinctive geometry changes where there is complex topography, such as near salt structures. Sedimentary bodies associated with linear, high-gradient channels often deposit within slope depressions as discreet J- or S-shaped structures in plan view. The dominant control on these sedimentary bodies is interpreted to be seafloor gradient and topography. This paper examines a number of these relatively young channels in terms of their geometry, gradient, levee development and seismic facies. The results improve our ability to predict subsurface channel geometries and recognise key evolutionary trends.

Distribution and dynamics of large woody debris and organic matter in a low-energy meandering stream

This field-based study, using a combination of bed sediment sampling and large woody debris (LWD) tracking, explores the storage of organic matter and mobility of LWD in Poplar Creek, a low-energy meandering river that contains a diverse arrangement of large woody debris (LWD). This research addresses the following questions: 1) Does LWD influence the spatial pattern of the storage of benthic organic matter in meander bends? 2) How mobile is the LWD in low-gradient meandering systems? 3) Do characteristics of LWD such as length/bankfull channel width ratio, affect movement? Based on detailed sampling of sediment and LWD monitoring, this study provides a detailed representation of the influence of LWD on the storage of benthic organic matter in meander bends, as well as a detailed documentation of the mobility of LWD in the study reach. This study has demonstrated that: 1) LWD strongly influences patterns of storage of benthic organic matter in low-gradient meander bends; 2) removal of LWD from meander bends in low-gradient systems can substantially reduce the storage of benthic organic matter; and 3) the LWD within the study reach at Poplar Creek is highly mobile. The high mobility of LWD, observed in this study, contrasts with the rates observed in high-gradient systems and suggests that LWD-influenced patterns of storage of organic matter are temporally variable and that storage of benthic organic matter occurs in pulses, interspersed with transport, that are keyed to the mobility of LWD. Although LWD may exert an equally significant control over patterns of storage of organic matter in high- and low-gradient systems, the differences in the mobility rates of LWD suggest a fundamental difference in the dynamics of LWD-mediated organic matter between low- and high-gradient systems.