Juneau — University and federal scientists told the City and Borough of Juneau Assembly’s Committee of the Whole that ongoing drone surveys and river monitoring show Suicide Basin’s storage capacity has been increasing even as the Mendenhall ice dam thins, raising the potential for large outburst floods in some years.

Dr. Jason Edmondson, a glaciologist with the University of Alaska, said the 2025 outburst “released 51,000 acre feet of water,” and that figure is more than half the volume of Mendenhall Lake. He described three main drivers: ice‑dam thinning, melt of floating icebergs in the basin, and small shifts in the ice‑dam location that can add or subtract thousands of acre‑feet of storage.

Why it matters: storage capacity and basin shape are core inputs to river forecasts, and they have changed substantially since routine drone surveys began in 2018. Edmondson said drone‑derived digital elevation models (DEMs) and repeated surveys allow scientists to build elevation–volume curves used by forecasters to estimate how much water the basin contains and how high the Mendenhall River could rise during an outburst.

Scientists’ findings and caveats

Edmondson presented a multi‑year series of hillshade DEMs and estimated that iceberg volume above water decreased from roughly 18,000 acre‑feet in 2020 to about 6,000 acre‑feet in 2025 — a ~12,000 acre‑foot gain in storage capacity attributable to iceberg melt alone. He said thinning of the ice dam and small positional changes in the dam contributed additional storage changes; combining those processes from 2020–25 produced an estimated net storage gain on the order of 7,000–10,000 acre‑feet.

Edmondson warned against simple extrapolation. “There’s a lot of variability from year to year,” he said, noting the physics of subglacial drainage and the condition of the subglacial system are not fully understood. He described conceptual elevation–volume curves that have shifted to the right (more storage at a given elevation) while the spillway elevation has lowered because of glacier thinning.

Operational forecasting

Aaron Jacobs of the National Weather Service said the elevation–volume curves from DEMs are “super, super important” to forecasting peak river stage. He reviewed the August 2025 event and said heavy rain and an atmospheric river made the hydrograph “dirty” — a mixture of rainfall runoff and an outburst flood — complicating the timing of the peak. Jacobs said laser surveys, in‑season drone imagery and USGS river gauges improved forecasts but that observation gaps during heavy weather reduced lead time and accuracy for the August event.

On interannual patterns and triggers

Both scientists told the assembly that observed years with large releases have distinctive hydrograph shapes, and that second or subsequent releases in the same season often behave differently from initial releases. Edmondson and Jacobs said the trigger for releases — whether overtopping, subglacial collapse, or a combination influenced by rainfall — remains a key scientific uncertainty.

What they will do next

Scientists said they will continue drone surveys (five to seven per year in recent seasons), laser scanning and collaboration across the university, USGS and the NWS to refine elevation–volume curves and improve scenario forecasts. Edmondson said the team will develop glacier‑flow models and keep refining in‑season projections for 2026 as more surveys become available.

Unresolved questions and local implications

Assembly members pressed whether mechanical thinning or engineered excavations of the ice dam would materially reduce flood risk; scientists replied such interventions could lower the spillway but would likely have limited effect relative to the basin’s depth and would be “not a lot of bang for your buck.” They emphasized that while volume trends point upward in recent years, annual variability — and coincident weather events — can make a given year much worse or milder than the simple trend suggests.

Evidence: presentation slides, repeated drone DEM imagery, and USGS/NWS hydrographs presented to the assembly.