(Reuters) - Chevron Corp shut a 29,400 barrel-per-day (bpd) pipeline carrying diesel and jet fuel between its Salt Lake City, Utah, refinery and Boise, Idaho after a leak was discovered in wetlands where millions of migrating birds will rest in the coming weeks, Utah officials said on Tuesday.

"One of the concerns we have is we have three weeks to clean up before the northern migration," said John Whitehead, assistant director of the Utah division of water quality.

The northern migration is the return of birds that wintered south of Utah to nesting areas north of Utah in Idaho, Montana and Canada.

"More important (than risk of fire) are the toxic effects on the ecosystem," he said. "The diesel is floating right now. That allows the cleanup to happen more easily. But we will be looking at longer term impacts to the wetland area it flows through."

The leak occurred in the wetlands less than a quarter of a mile from Willard Bay, which drains into the Great Salt Lake, near Ogden, Utah.

There was no visible sign that contamination has entered the bay, said Whitehead. A small surface stream flows near the pipeline break and that flows directly into the bay.

Willard Bay is a manmade freshwater reservoir sandwiched between the Great Salt Lake and the Wasatch Mountains. The bay and adjoining wetlands are a key stop for migratory water birds like ducks, geese, tundra swans, white pelicans and shore birds.

Chevron detected a drop in pressure along the pipeline at 2:30 p.m. MDT Monday (1830 GMT Monday) and closed the valves allowing diesel and jet fuel to flow though the line.

"There are no further details about the impact on (Chevron) operations at this stage. The volume of the release is still to be determined," Chevron spokesman Gareth Johnstone said in an email.

The leaking pipeline carries diesel and jet fuel from the 45,000 bpd Salt Lake City refinery through Burley, Idaho to Boise. A second pipeline carries gasoline from Salt Lake City to Boise.

Whitehead said the leak was estimated to be between 100 barrels and 150 barrels (4,200 gallons-6,300 gallons).

The site of the leak was found at 6:30 p.m. MDT Monday.

The company said it had mobilized emergency and clean-up crews, and initiated emergency response procedures.

About 100 people were working on containment and clean-up, Whitehead said.

In 2010, a leak from an oil pipeline supplying Chevron's Salt Lake City refinery contaminated a creek feeding the Great Salt Lake. Clean-up workers collected about two-thirds of estimated 785 barrels (33,000 gallons) in oil that leaked from the pipe.

(Reporting By Laura Zuckerman in Salmon, Idaho and Shruti Chaturvedi in Bangalore; Editing by Himani Sarkar)