The Effects of Sandbar Formation and Inflows on
Aquatic Habitat and Fish utilization in Pescadero,
San Gregorio, Waddell and Pomponio Creek.
Estuary/Lagoon Systems, 1985-1989

Jerry J. Smith
Department of Biological sciences
San Jose state University
San Jose, CA 95192
21 December 1990

Report Prepared Under Inter-agency Agreement 84-04-324
Between Trustees for California State University and the California Department of Parks and Recreation.

Abstract

The fish habitat quality of the small lagoon/estuaries at
Pescadero, San Gregorio and Waddell creeks was generally good,
but limited in extent, when the stream mouths were open to full
tidal mixing in winter and early spring. In late spring, summer
and fall, habitat quality depended primarily upon the timing of
sandbar formation and breaching and upon the quantity and quality
of freshwater inflows to the lagoons after sandbar formation.
Adequate inflows to these shallow lagoons after bar formation
resulted in rapid conversion of the lagoons to unstratified fresh
water, relatively cool water temperatures, high dissolved oxygen
levels and high invertebrate abundance. Low inflows to the
lagoons, due to late sandbar formation, drought or upstream
diversions, resulted in delay or failure of salinity
destratification. Saline, stratified lagoons acted as solar
collectors and had higher water temperatures, especially within
the more saline bottom waters. Stratified lagoons also often’ had
low bottom dissolved oxygen levels and reduced invertebrates.
Summer or early fall natural or artificial sandbar breaching
was usually rapidly followed by sandbar re-formation; resulting
in stratified, saline, warm and unproductive lagoons: However,
in the summer of 1989 the sandbar at Pescadero Lagoon
remained open for several months after artificial sandbar
breaching. Although the upstream portions of the estuary were
shallow, stratified and warm, the well-mixed main embayment was
cool and well-oxygenated. Fish populations utilizing the lagoons consisted;
of freshwater, estuarine and saltwater species. The number of
species increased with lagoon size. The highest diversity
occurred at Pescadero Lagoon in early summer of 1986, when many
juvenile saltwater fishes were present in the open lagoon. After
lagoon conversion towards freshwater conditions, following
sandbar closure, many saltwater species declined or disappeared.
Many species also declined or disappeared in the warm, saline,
unproductive lagoons associated with the drought years.
Juvenile steelhead survival and growth was excellent when the
lagoons were open to full tidal mixing and when the closed
lagoons were converted to fresh water. Growth was poor during
long, stratified transition periods between sandbar closure and
conversion of the lagoons to fresh water. Survival was poor
during periods of prolonged warm, stratified conditions. The
high numbers and/or large sizes of steelhead reared in the
lagoons during years of freshwater conversion demonstrate that
these lagoons can potentially contribute the majority of
steelhead smolts produced in these small coastal watersheds.

Managing these lagoons for production of juvenile steelhead requires:

1) prevention of artificial summer sandbar breaching; and
2) insuring sufficient inflows after sandbar formation to rapidly
convert stratified, saline lagoons to fresh water.

Introduction

The annual summer drought in California results in sharp
declines in stream flows in coastal streams. For smaller streams,
declining stream flow and summer beach development result in
development of a sandbar which dams the stream mouth to produce
a lagoon. These lagoons may provide warm, deep-water areas for
swimming and boating and habitat for fish and wildlife. The
raised water levels behind the sandbar can also flood adjacent
lands, producing valuable wetlands and/or threatening
agricultural or urban developments. Despite recent interest in
wetlands and estuaries, relatively few studies have been done on
California central coast lagoons, although they were long ago
shown to be important for steelhead (Oncoryhnchus rnykiss) and
salmon (0. kisutgb) (Shapovalov and Taft 1954). Lagoons and
their associated wetlands have been actively managed by diversion
of inflow waters for agricultural and municipal uses, diking of.
surrounding land, and artificial breaching of the sandbar for
flood, odor, and insect control. This report summarizes the
results of studies undertaken for the California Department of
Parks and Recreation to determine habitat dynamics and fish
utilization ‘in four small coastal lagoons. The goals of the
studies were: A) to provide information to guide the restoration
and management of wetlands and the estuary/lagoon at Pescadero
Marsh Natural Preserve (NP) r and B) to provide information on
sandbar management and lagoon inflows necessary to maintain
aquatic habitat in the face of upstream diversions and drought
conditions.

Check out Part 1 of the full report in pdf format here.

Check out Part 2 of the full report in pdf format here.