Back in the 1920s, well-to-do city dwellers began to build summer homes along the numerous rivers and creeks that indent Anne Arundel County's shoreline, assured of tranquil boating.

All that has changed. As the county has become increasing popular with vacationers as well as commuters from both the Washington and Baltimore areas, the rivers and creeks have become so crowded with pleasure boats that on weekends, as one resident complained, "you could walk short to shore across the boats."

Yesterday the Anne Arundel County Council, citing "extreme" congestion imposed an 11-month moratorium on marina construction and expansion. Its purpose is to gain time to prepare zoning controls aimed at minimizing the enviromental, water safety and recreational threats caused by the water traffic.

The problem, as county authorities view it, is the result of Ann Arundel's increasing popularity as a place to live. Dozenz of shoreline communities, offering boat slips in their real estate packages, have sprung up along the 431 miles of shoreline on the Chesapeake Bay, four major rivers and several large creeks.

Already 60 percent of the shoreline is developed and the watermen who make their livelihoods on the rivers and creeks have all but disappeared. During the 1970s, Anne Arundel has grown faster than either Montgomery or Prince George's county, and much of the influx has come from people working in metropolitan Washington.

Originally County Executive Robert A. Pascal proposed a moratorium only on commercial marinas, frequently rented by out-of-county residents who find Anne Arundel their quickest access to the bay. But the council approved a more sweeping bill that covers all marinas, including those serving new shoreline communities.

"The weekends have gotten so bad on these waterways that frankly I never go out then anymore," said E. Gordon Rilep, president of the Magothy River association, one of several citizens groups supporting the moratorium. "Everybody should have a right to the water, but there's a point at which it becomes too crowded."

The congestion has brought shore erosion, water pollution and recreational hazards as sailboats and speedboats jockey for position in the narrow river channels leading to the bay.

"We need to protect the water from becoming a parking lot of boats," said Nancy Kelly, staff biologist of the Chesapeake Bay Foundation, which has urged better management of the county's waterways.

On the other hand, residents of at least three shoreline communities objected to the moratorium because it prevents the building of new piers they have been planning for years.

"The residents of our community moved here for the primary purpose of year-long use of the water, and now, it will be denied some of them," said Stuart Marder, president of Cape St. Clair Improvement Association.

Marder, principal of North Bethesda Junior High School, moved to the 2,000-home Cape St. Clair neighborhood on the Magothy River in 1972. A total of 175 boat slips have been built there, and the association was about to construct 26 more at a cost of $27,000 to the residents. "Right now we have a waiting list of 75 people who want slips, and that issue more than anything else is what my telephone rings about at night," Marder said.

The Avocet Corp., which has spent eight years assembling permits to develop a community with a 500-slip marina on Bodkin Creek, persuaded a Circuit Court judge yesterday to order the county to show cause why it should not issue the needed marina building permits. Avocet officials told Judge Bruce C. Williams that financing of the entire project is contingent on the marina construction.