LINKING FLUVIAL SEDIMENTATION CHARACTERISTICS TO LARVAL DRAGONFLY HABITAT

The Huron Mountains in Michigan’s Upper Peninsula contain a wide variety of river forms creating habitat for diverse aquatic life, including burrowing dragonflies (Odonata: Gomphidae). Over summers in 2019 and 2020, field work focused on characterizing river sediment, flow properties, and water quality as well as documenting aquatic Gomphid larvae associated with sediment conditions. In 2019, focus was on fluvial characteristics of the diverse fluvial environments, done via Wolman pebble counts and bed sediment sampling. In 2020, geomorphic analysis of river dimensions and water quality were added, as well as a focus on water quality and invertebrate samples. Co-located with geomorphic surveys, a Surber sampler was deployed in a single transect perpendicular to the flow to collect five replicate benthic invertebrate samples.

All rivers showed exceptional clarity and high water quality. All but one site had nitrate levels below 0.5mg/L, while Phosphate measurements averaged 1.61mg/L. Total suspended solids (TSS) were negligible at all sites. In terms of sediment, the riverbeds varied at each site from extra-fine sand (such as in the lower Salmon-Trout River) to large boulders and significant bedrock (Styx River). Bed material of the streams showed clear linkage to its provenance, generally either from the Jacobsville sandstone or Precambrian metamorphic rock. As the longest river in the study region, the Salmon-Trout River, with typical meandering geometry, showed a significant downstream increase in sand percentage. Rivers that contained larger rocks and coarse sand bed load have the potential to provide more habitat for larval Gomphids. Streams with higher gradients and coarser average grain size appeared to hold more emergent cobble and boulders, such as in the Pine River and Rush Creek. Rush Creek, in particular, had a bimodal clast distribution with peaks in coarse sand and medium-to-coarse gravel. This distribution seems to provide ideal habitat for aquatic Gomphid larvae, with sand for burrowing and larger clasts for protection. Gomphid larvae were recorded at four of nine sites with the highest abundance at Rush Creek, despite Rush Creek being one of the smaller rivers sampled.