That big free-agent pool of top-tier wide receivers that was going to be available for the 49ers has suddenly drained down to the size of a kiddie pool.

There remain some good catches available, but the competition and cost for them likely will be high when the free-agency period opens March 13.

With Monday being the deadline for teams to put the franchise tags on potential free agents, several teams in recent days have done just that on marquee wide outs.

The Patriots tagged Wes Welker. The Chiefs tagged Dwayne Bowe. Also staying put are the Eagles’ DeSean Jackson and the Bills’ Stevie Johnson.

With only two wide receivers under contract for 2012, the 49ers need to build their receiving corps and add a complementary, impact wide out to play opposite the lone incumbent starter, Michael Crabtree.

The top two wide receivers still available are the Chargers’ Vincent Jackson and the Saints’ Marques Colston.

Jackson, at 6-foot-5 and 232 pounds, not only is a big target – effective on third downs and in the red zone – but a big-play receiver as well, averaging 18.4 yards per catch in 2011 with nine TDs. However, as Matt Barrows of the Sacramento Bee notes, bidding for Jackson is expected to be fierce, beginning at about $10 million per year. At 29, he’s also a year older than Colston.

Colston, who’s been the Saints’ leading receiver the past three seasons, also is a big target at 6-foot-4 and 225 pounds. This past season, he had 80 catches for 1,143 yards and eight TDs. Barrows notes Colston has had injury issues but is “very much a 49ers-type player in that he’s reliable and hard-working, and he doesn’t mind doing the blocking and other dirty work required at his position.”

The Steelers’ Mike Wallace, just 25, ranks at the same level as Jackson and Colston in terms of talent and production – if not higher. He’s one of the speediest players in the NFL. But, he’s a restricted free agent, not unrestricted, meaning the 49ers would have to not only offer him an enticing, expensive deal, but have to compensate the Steelers for their loss if they signed him to an offer sheet and Pittsburgh didn’t match it. That price would be their first-round pick, the 30th overall this April.

Other unrestricted free agents available include the Giants’ Mario Manningham, Rams’ Brandon Lloyd, Colts’ Pierre Garcon and Reggie Wayne, Saints’ Robert Meachem, Cowboys’ Larent Robinson, Bears’ Roy Williams, Raiders’ Chaz Schilens and Jets’ Plaxico Burress.

As Eric Branch of the San Francisco Chronicle points out, both Colston and Jackson fit into the Niners’ desire to get a physically large receiver as well as one who is effective. That’s one reason the Niners signed free-agent veteran Braylon Edwards last summer.

“He is a big receiver,” coach Jim Harbaugh told Branch at the time. “We were looking for that big-stature receiver.”

With interest in Jackson and Colston high from several teams, it’s possible the 49ers could deem both to be too expensive.

“With the pool of top-end wide outs drying up, Jackson and Colston will likely command even larger contracts,” writes Branch. “In other words, they won’t be anywhere near the sale rack or bargain bin, popular free-agent shopping spots for the Niners.”

The 49ers also must decide if they want to re-sign two of their own, Ted Ginn Jr. -- also an outstanding return man -- and Joshua Morgan, who was injured at mid season. Indications are the 49ers will keep both.