How Scientists Mapped the Elwha's Transformation Through Bathymetry and Topography
Documenting the largest dam removal in U.S. history through comprehensive 2011 surveys
In September 2011, engineers began an unprecedented environmental experiment: dismantling two century-old dams on Washington's Elwha River.
For nearly 100 years, the Elwha and Glines Canyon dams had stood as concrete barriers, disrupting natural processes and blocking migratory salmon from their spawning grounds. As the largest dam removal project in U.S. history commenced, scientists faced a critical question: How would the river respond to its newfound freedom?
That same month, a team of U.S. Geological Survey researchers conducted a comprehensive topographic and bathymetric survey of the Elwha River delta—creating a crucial snapshot of the river's form at this historic turning point 1 2 .
This dataset would become the essential baseline against which all future change would be measured, capturing the river's profile just as millions of cubic yards of sediment were about to be released.
Elwha Dam completed
Glines Canyon Dam completed
Dam removal begins
Dam removal completed
The Elwha River winds 72 kilometers from the snow-capped Olympic Mountains to the Strait of Juan de Fuca, draining a steep, mountainous landscape of over 800 square kilometers 2 5 . Prior to dam construction, the river was renowned as one of the most productive salmon streams in the Pacific Northwest, supporting vibrant populations of all five Pacific salmon species alongside steelhead and other native fish 6 .
The completion of Elwha Dam in 1913 and Glines Canyon Dam in 1927 fundamentally altered the river's ecosystem. The dams:
Salmon habitat reduction due to dam construction
By the time dam removal began, the coastal delta was starved of sediment, salmon populations hovered at critically low levels, and the river's connection to the sea existed in name only 2 . The stage was set for one of the most ambitious restoration projects ever attempted.
To document the Elwha River delta at this pivotal moment, USGS scientists implemented an integrated survey strategy in August 2011 that captured both underwater and terrestrial topography 1 . The team recognized that understanding the river's transformation would require precise measurements across the entire system—from the river channel to the coastal zone.
Used personal watercraft with echosounders to map underwater surfaces 1
Recorded using GNSS systems on rover poles to capture shoreline slopes 3
This multi-faceted approach allowed scientists to create a seamless digital elevation model that bridged the gap between land and sea—a crucial perspective for understanding how sediment would redistribute across the entire system 1 .
| Equipment | Function | Application in Elwha Research |
|---|---|---|
| Single-beam Echosounder | Measures water depth by emitting sound waves and timing their return | Mounted on personal watercraft to map river and nearshore bathymetry 1 |
| GNSS Receiver | Provides precise location data using global navigation satellite systems | Used for both topographic and bathymetric positioning with centimeter-level accuracy 1 |
| Real Time Kinematic GPS | Enhances GPS precision using a fixed base station and mobile rover | Employed for beach profile surveys in 2011, achieving elevation accuracy of 0.03-0.05m 3 |
| Instrumented Tripod | Deploys multiple sensors in the bottom boundary layer | Measured current velocity, suspended sediment, and seabed changes offshore |
| Acoustic Doppler Velocimeter | Measures water current velocity in three dimensions | Deployed on tripods to characterize tidal currents and sediment transport |
The August 2011 survey provided scientists with a crucial pre-removal baseline that revealed several important characteristics of the dam-impacted system:
The digital elevation models showed a sediment-starved delta that had been eroding for decades due to interrupted sediment supply 1 6 . The elevation data documented the exact dimensions of this erosion, creating a reference point against which future sediment accumulation could be measured.
The bathymetric and topographic data also established the existing channel morphology of the lower Elwha River—an anabranching system characterized by multiple channels separated by bars and vegetated islands 5 . This complex channel form represented the river's adaptation to decades of reduced sediment load and altered flow regimes.
Estimated sediment volume trapped behind Elwha dams prior to removal
| Characteristic | Pre-Dam Removal Condition | Significance |
|---|---|---|
| Sediment Supply | Trapped behind dams (19 million m³) | River downstream was sediment-starved 2 |
| Coastal Erosion | Documented erosion of delta shoreline | Provided baseline to measure future accretion 1 |
| Channel Form | Anabranching pattern with multiple channels | Represented river's adaptation to altered sediment regime 5 |
| Salmon Habitat | Limited to lower 8 km of river | Restored access to 145 km of habitat after dam removal 2 |
Perhaps most importantly, the 2011 data captured a landscape on the verge of transformation. The elevation models documented the final configuration of the reservoirs themselves—Lakes Aldwell and Mills—which held an estimated 19 million cubic meters of sediment that would soon begin moving downstream 2 . The surveys precisely quantified the storage capacity that had been lost to sedimentation and provided crucial information for predicting how sediment would release during dam removal.
The beach profiles collected in May 2011 3 documented the exact shape of the shoreline before the sediment wave arrived. This included measurements of beach elevation, slope, and the position of the shoreline—all of which would change dramatically in the coming years as river-borne sediments reached the coast.
The August 2011 survey was just the beginning of an extensive monitoring program that would track the Elwha's transformation for years to come. As dam removal progressed between 2011 and 2014, the river did not disappoint—releasing massive quantities of sediment and beginning a dramatic reshaping of the landscape 2 .
The scientific foresight to collect comprehensive baseline data created unprecedented research opportunities:
Tracking as an estimated 7-8 million cubic meters of sediment transported downstream from the former reservoirs 2
Monitoring as renewed sediment supply reversed decades of erosion, rebuilding beaches and nearshore habitats 6
| Sediment Characteristic | Pre-Removal Projection | Post-Removal Findings |
|---|---|---|
| Total Volume Trapped | 19 million cubic meters | 19 million cubic meters 2 |
| Expected to Transport | 7-8 million cubic meters | Approximately 8.2 million tonnes (suspended sediment) released by Sep 2013 |
| Fine-Grained Sediment | 50-67% of total (silt, clay, sand) | Confirmed dominance of fine-grained material in suspended load 2 |
| Coarse-Grained Sediment | 25-33% of total (gravel, cobbles) | Documented gravel and cobble redistribution to river channel and delta 2 |
The tidally dominated environment of the Elwha River delta proved particularly effective at distributing sediments along the coastline, with research showing that tidal currents—rather than wave action—initially controlled the dispersal of fine-grained materials . This finding highlighted the importance of understanding local oceanographic processes when predicting the outcomes of large river restoration projects.
The bathymetry and topography data collected from the Elwha River delta in August 2011 represents far more than technical measurements—it captures a river at the threshold of renewal.
This comprehensive dataset provided the essential baseline that has allowed scientists to document one of the most dramatic river transformations ever studied, creating a valuable reference for future dam removal projects worldwide.
The Elwha experiment has demonstrated that given the opportunity, rivers can reclaim their dynamic nature. The systematic surveys conducted before, during, and after dam removal have given scientists unprecedented insight into how landscapes respond when fundamental processes are restored.
As salmon once again swim into the upper reaches of the Elwha watershed and sediments rebuild eroded coasts, the 2011 baseline data continues to serve as the foundational record of a river reborn—a testament to the power of integrated science in guiding ecosystem restoration.