* Backlog of more than 100 idled vessels reduced to 75
* All traffic to stop Tuesday evening, resume Wednesday morning (Updates with resumption of inbound ship traffic, backlog cut to 75 vessels, adds byline)
By Kristen Hays
HOUSTON, March 25 (Reuters) - The Houston Ship Channel, a critical waterway for oil shipments, gradually reopened on Tuesday after a four-day closure that forced at least one big refiner to cut output and caused a backlog of more than 100 vessels.
By Tuesday afternoon, the backlog had been reduced to 75 vessels.
The channel, which shut on Saturday because of an oil spill, reopened first for outbound and later for some inbound ship traffic, according to the U.S. Coast Guard and Houston Pilots.
The Intracoastal Waterway also opened to traffic in stages, first eastbound and later westbound, the Coast Guard said. Authorities had earlier allowed limited barge traffic to resume.
The unusually lengthy closure of the waterway, which connects the Gulf of Mexico to inland terminals and facilities, halted imports of crude to refiners that produce more than one-tenth of the country's fuel and delayed exports of some products. More than 100 large vessels, many of them oil tankers, had been queuing up on either side of the channel.
More than a dozen tankers carrying some 9 million barrels of crude were among those held up by the shutdown.
Capt. Clint Winegar, head of the Houston Pilots, said Tuesday afternoon that the Coast Guard gave his crew "a small window" to let a few waiting ships sail into the channel during daylight after outbound ships started moving earlier in the day.
He said 29 outbound ships were waiting to leave and 46 were waiting to enter the channel.
Winegar said both inbound and outbound movements would stop at 6 p.m. CDT on Tuesday, and resume at 7 a.m. CDT on Wednesday.
The Channel, as well as offshoots to Texas City and Galveston Bay, shut on Saturday after an oil barge collided with a cargo ship and spilled about 4,000 barrels, or 168,000 gallons (636,000 liters), of heavy fuel oil.
Winegar said the first to leave on Tuesday was a cruise ship at the Bayport Cruise Terminal between Houston and Galveston Bay. Ships moving both ways included tankers carrying oil or refined products, cargo ships and cruise ships. Vessels with low fuel had first priority when inbound movements began, he said.
Coast Guard Capt. Brian Penoyer, commander of the cleanup operation, has called the spill "significant." The Galveston Bay spill is far smaller than the 260,000 barrels, or 11 million gallons, of crude oil that was released when the Exxon Valdez struck a reef in Prince William Sound in 1989.
The Coast Guard also had several decontamination and inspection stations for ships entering and exiting the channel.
Before traffic reopened on Tuesday, 54 ships waited to enter the channel headed to the port of Houston, while 47 waited to leave, according to the Coast Guard.
On a typical day, 60 to 80 large vessels, including tankers, freighters, containers and cruise ships, and 300 to 400 tugs and barges move through the channel, according to Penoyer.
Since the channel was shut, the nation's second-largest refinery, Exxon Mobil Corp's 560,500-barrel-per-day (bpd) facility in Baytown, Texas, on the east side of Houston, has had to cut production.
The Kirby Inland Marine barge, which partially sank after the collision, was emptied of all remaining fuel oil and refloated by Monday, according to Kirby Inland Marine.
A total of 70 vessels were collecting heavy fuel oil from the bay. Some of the oil has been pushed by north winds into the Gulf of Mexico, the Coast Guard said. The oil has also been washing ashore.
U.S. and Texas wildlife officials said on Tuesday that 10 birds were confirmed dead from the oil spill, and 18 birds covered with oil were captured for cleaning.
Reporting by Erwin Seba and Kristen Hays; Editing by Jeffrey Benkoe and Jan Paschal