is in early stages of determining viability for FIRO-related changes to operating procedures and more
effective management of the reservoir by leveraging improvements in hydrology and atmospheric
sciences. The Dam and reservoir are situated on the Green River in the central Cascade Mountains in
Western Washington and are operated by the US Army Corps of Engineers (USACE) in Seattle. The
reservoir behind the dam has several functions: It is the main water supply for the city of Tacoma and is
filled during the summer months, and it is empty during winter months when it is used for flood risk
management during AR events, protecting billions of dollars of infrastructure downstream. It is often
necessary to balance those functions with environmental concerns related to salmon. The reservoir is
maintained by the Cascade Mountains’ abundant winter snowpack and precipitation and is currently
completely refilled every year.
Since the reservoir behind Howard Hanson Dam must be empty before the flood season starts and is
reliant on snowpack and precipitation to fill in late spring, impacts in precipitation and temperature due
to climate change are important for how the USACE operates and manages flood risk and water supply
in the future. Future concerns of insufficient flows for fish, timing of runoff for refill, potential for
decreased winter snowpack, the possibility of adding storage space to the reservoir, and future changes
in extreme precipitation, makes climate change an essential piece of the FIRO equation. This work sets
the stage for the coming years and will outline the importance of the inclusion of in-depth research on
each of the above interests in the Green River Basin.
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