Measuring Gravel Bar Mobility In A Large River With Tracer Gravel

The Bureau of Reclamation deployed 600 tracer clasts labeled with Passive Integrated Transponder (PIT) tags on gravel bars in the Methow River in the vicinity of the Sugar Levee near Twisp, WA. PIT tags equip each rock with a unique identifier that can be detected and read from a few feet away via Radio Frequency Identification (RFID) technology. The purpose of the experiment was to test a hypothesis that the Sugar Levee is disrupting sediment transport dynamics through the study reach and causing ‘excess’ deposition on a gravel bar downstream of the levee, resulting in bank erosion and property loss on the opposite bank.

The tracer rocks were installed on 4 gravel bars upstream of the levee and on 1 bar across the river from the levee in October 2018. Searches for the tracers in 2020 and 2021 recovered and surveyed the locations of 448 (75%) and 356 (59%) of the 600 rocks installed. Of the rocks installed on the furthest upstream bars (assigned the names Bar 0, Bar 1, Bar 2), 80% remain on the bar where they were installed. In contrast, 45% of the tracers installed on the bar immediately upstream of the levee (Bar 3) and 87% of the tracers installed on the bar across the river from the levee (Bar 4) have been eroded and transported downstream. Some tracers have traveled more than a mile.

We found a weak negative correlation between tracer size and tracer mobility, but it does not appear to explain the differences in tracer mobility. Instead, an increase in bed slope at the upstream end of Bar 3 continuing to the downstream end of the levee appears to be responsible for the enhanced sediment mobility adjacent to the levee. It seems likely that confinement by the levee has caused the river to incise adjacent to the levee, resulting in the increased steepness.

No tracers from Bars 0, 1, or 2 were found downstream in 2021. Tracers eroded from those bars have disappeared. They may be trapped in areas that were too deep or swift to search by wading or they may have been transported even further downstream.

All but three of the tracers found downstream of Bar 4 were installed on Bar 4. This over-representation of tracers from Bar 4 found downstream is also unexplained. This may be partially an artifact of the relatively low recovery rates but that seems unlikely to be the full explanation. A third tracer recovery scheduled for Oct. 2022 may or may not resolve these unexplained results.