Living Earth Biology · Human Impacts Project

Human Impacts on
Lake Berryessa

Monticello Dam & the Putah Creek Ecosystem

By Andrew Taylor

Location

Where is Lake Berryessa?

Vaca Mountains, Napa County — California's 7th-largest man-made lake, fed by Putah Creek.

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Napa County, CA

East of the Napa Valley wine region.

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Vaca Mountains

Reservoir held by Monticello Dam.

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Putah Creek

Flows downstream through Davis.

Oak-studded California foothills above a blue reservoir at golden hour

Ecosystem

An oak-studded watershed

A Mediterranean climate of dry summers and wet winters supports a mosaic of habitats.

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Oak woodland

Blue and valley oaks on the slopes.

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Chaparral

Drought-tolerant scrub on dry hills.

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Riparian forest

Cottonwoods line Putah Creek.

Sun-dappled oak woodland in California foothills

Biodiversity

Native inhabitants

Putah Creek once supported some of California's most iconic native fish and birds.

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Native fish

Chinook salmon, steelhead, Pacific lamprey.

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Birds

Ospreys, herons, riparian songbirds.

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Mammals

River otters, gray foxes, deer.

Salmon swimming upstream in a cold rocky river

The Problem

A wall across the watershed

Monticello Dam blocked fish migration and inverted Putah Creek's natural flow regime.

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Built 1953–1957

304-ft concrete arch dam.

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Migration cut

Salmon and lamprey blocked upstream.

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Flow flipped

High summer flow, low winter flow.

Massive concrete dam wall holding back a deep blue reservoir

Cause & Timeline

When did it start?

Construction finished decades before environmental impact laws were on the books.

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1957

Monticello Dam completed.

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Pre-NEPA

No environmental review required.

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~70 years

Of ongoing ecological disturbance.

Aerial view of a circular spillway funnel draining lake water

Affected Organisms

Who lost?

Migratory fish and streamside birds bore the brunt of the disturbance.

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Chinook salmon

Extirpated upstream of the dam.

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Pacific lamprey

Blocked from ancestral spawning beds.

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Riparian birds

Lost streamside breeding habitat.

Healthy flowing creek lined by green riparian forest

Invasives

Who took over?

Warm, slow reservoir water favored non-native fish that natives couldn't outcompete.

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European carp

Stir up sediment and displace natives.

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Bass & sunfish

Stocked for recreational fishing.

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Catfish

Thrive in warm reservoir conditions.

Calm reservoir surface reflecting clouds

Biodiversity Impact

Native diversity collapsed

Without cold, free-flowing water, sensitive native species lost the conditions they evolved in.

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Native fish

Counts plunged after the dam closed.

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1987–1992 drought

Dry creek killed fish near Davis.

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Bird diversity

Reduced as wetlands shrank.

Cracked dry riverbed during California drought

Impact on People

Why it matters to us

The dam supports a half-million people, but it also costs communities downstream.

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500,000 people

Solano residents drink this water.— Bureau of Reclamation

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Lost fisheries

Salmon collapse hurt commercial fishing.

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Tribal impact

Salmon are culturally vital to tribes.

Calm rural drinking-water reservoir at sunrise

Why It's a Problem

Disrupted rivers unravel

Dams break the flows of energy, nutrients, and genes that hold river ecosystems together.

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Nutrient cycling

Sediment and minerals trapped behind the dam.

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Gene flow

Salmon populations isolated from each other.

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Trophic cascade

Losing top fish reshapes the whole food web.

Clear river flowing over rocks in a forest

Did You Know?

A town beneath the lake

Filling the reservoir erased an entire valley and the historic town of Monticello.

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Town flooded

Monticello submerged in 1957.

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300 graves

Relocated before flooding.— Napa Valley Register

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Glory Hole

72-ft funnel spillway visible during overflow.

Worldwide

It's not just here

Large dams disrupt ecosystems around the world — sometimes restoring rivers reverses the damage.

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Three Gorges Dam

Chinese sturgeon nearly extinct after closure.

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Elwha Dam removed

Salmon returned in just a few years.

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Aswan High Dam

Nile sardine fishery collapsed downstream.

Past Solution

The Putah Creek Accord (2000)

A court-ordered agreement guaranteeing minimum year-round flows downstream of the dam.

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Rationale

Steady baseline flows keep aquatic life alive.

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Benefit

Salmon returned; birds doubled by 2015.— UC Davis

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Challenge

Dam still blocks upstream migration.

Healthy flowing creek surrounded by green riparian forest

My Solution

Fish ladder + updated Accord

Build a bypass around the 304-ft dam wall and raise fall migration flows.

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