TRANSPORT OF GRAVEL BY THE MILL RIVER ON SMITH COLLEGE CAMPUS IN NORTHAMPTON, MASSACHUSETTS: INSIGHTS FROM RFID (RADIO FREQUENCY IDENTIFICATION) OF TAGGED PEBBLES

In Oct./Nov. 2019, four pebbles (7-13 cm in diameter, weighing 0.4-1.8 kg) were marked using small Passive Integrative Transponder (PIT) tags (23 and 32 mm long, <4 mm in diameter) on a small mid-river gravel bar (~3 m wide, 8 m long) downstream from the Paradise Pond dam and the pedestrian Lamont Bridge over the Mill River on Smith College Campus. The goal of the project was to set the stage for long-term monitoring of gravel transport within the larger scale interdisciplinary Mill River Watershed Assessment Project.

During a high flow event on Dec. 14, 2019, the gravel bar was completely submerged. Maximum velocity of 1.23 m/s was measured by a flowmeter held near the surface, ~1.5 m from the river bank adjacent to Lamont Bar. Average velocity of 0.82 m/s was recorded at the Lamont Bridge by the Mill River Monitoring Project gage station (total measured discharge/measured cross-sectional area). RFID relocation of tagged pebbles in Feb. 2020 indicated that 2 pebbles rolled downstream, but were not transported a significant distance. Minimum calculated flow velocities required to initiate transport of pebbles were 0.9-1.5 m/s (equations from Nandasena et al. 2011), but the Dec. 2019 event was not strong enough to cause significant transport, especially if pebbles were shielded by vegetation and larger angular basalt pieces dumped here during the construction of the dam, the bridge and the nearby flood-control levee.

In the meantime, Smith College began an annual sediment redistribution via mechanical equipment into the river channel within the Paradise Pond. Some sediment is transported by river flow through the dam sluice, settles within the plunge pool below the spillway, and passes further downstream settling in the river below Lamont Bridge where it can be trapped by vegetation on the gravel bar. RFID relocation of all 4 tagged pebbles in Dec. 2022 indicated that they have not moved substantially in the last 3 years. Despite the use of a narrower-range pin-point radio antenna, only 1 pebble was visually located. The remaining 3 were submerged in water and/or buried under new sediment and vegetation. Their relocation will be attempted during lower water levels in summer 2023 to more specifically evaluate the dynamics of fluvial sediment transport in and around this vegetation-stabilized and anthropogenically-impacted gravel bar.