Work

Tributary-to-Mainstream Aquatic Macroinvertebrate Discontinuities in the Colorado River, Southwestern USA

Year: 2026

Type: article

Abstract: Tributary-to-mainstem discontinuities (TMDs) are understudied, but are likely common in river networks, arising from abrupt transitions in stream order and dominant ecological factors. We present a conceptual model of aquatic macroinvertebrate (AMI) TMD directionality and relative magnitude by contrasting the impacts of hydrography, geochemistry, and sediment transport on tributary-related channel-floor precipitate cementation and the mainstream embeddedness (burial) of channel-floor substrata in fine sediment. We test that model using AMI assemblage density/m2, species richness/sample, and diversity data from 24 tributaries confluent with the regulated Colorado River in Grand Canyon through pairwise and multivariate analyses of long-term discharge records and substrate and water-quality data in three habitats: tributaries, their confluences, and adjacent mainstream habitats. Mean AMI density decreased 2.7-fold from low to high cementation, 6.1-fold from low-to-high embeddedness, and 136.0-fold across combined gradients. We also analyzed pre-dam aquatic insect literature, finding that TMDs were naturally common in Glen Canyon upstream but were more strongly tributary-positive (discontinuity magnitude, Dmag = 0.62 in pre-dam Glen Canyon) compared to tributaries in the post-dam Grand Canyon (Dmag = 0.31). We conclude that, depending on Dmag directionality, tributary confluences can function as hotspots or barriers to AMI assemblage development. Our results demonstrate that TMDs are and were common in the contemporary regulated and natural unregulated Colorado River corridor, and we expand the concept of biotic discontinuity to improve understanding of fluvial ecosystem ecology and constraints on river and dam management.

Source: Water

DOI: 10.3390/w18030395

Authors: Lawrence E. Stevens, Joseph H. Holway, Craig Ellsworth

Language: English

Topic: Freshwater macroinvertebrate diversity and ecology

Subfield: Ecology

Field: Environmental Science

Domain: Physical Sciences

Open Access status: gold

Estimated APC paid: $3,137

Funders: U.S. Department of Energy, U.S. Geological Survey, National Park Service, Argonne National Laboratory

Award: Machine Learning Assisted Classification of Ca-Organic Matter Complexation Using X-ray Spectroscopies