Go past the wooden gate and turn right into the only open doorway.
Step off the edge and slide down the ramp onto the landing below. Turn left.
Step—don't jump—off the edge onto the slope below. Then immediately jump forward to clear the spikes. (Or, hang from the edge, drop onto the slope, and backflip over the spikes.) Drop down into the dark passageway below.
Light a flare if you can and turn to face the crawlspace on the right. There's another poison-breathing lizard inside. If you don't want to take damage, crawl forward just until Lara's hands and upper body enter the tunnel. This triggers the lizard. Then immediately back out of the opening.
Draw the shotgun or Desert Eagle...
...and hop back firing as soon as Lara targets the lizard. You should be able to kill it before it spews poison.
Now go through the crawlspace and climb the blocks ahead.
Cross the wooden bridge spanning the river, enter the room ahead, and follow the passage on the left...
...to a green ledge overlooking the water. Jump over to the stone archway covered with greenery to get another save/power-up crystal.
Turn around and kill the lizard in the doorway where you came from before jumping back.