CIUDAD TECÚN UMÁN, Guatemala — Traffic on the international bridge that spans the Suchiate River and links the Guatemalan town of Ciudad Tecún Umán with Mexico, is often light. For the immigration and customs agents posted at each end of the bridge, things are quiet, even sleepy.

But from the bridge, you can see a flurry of activity on the river throughout the day, everyday, with scores of crude rafts crisscrossing the waterway, carrying people and cargo from one side to the other.

The bridge is the legal route; down below is the illegal way.

While Mexico’s northern border with the United States is fortified with elaborate barriers and patrols, the Guatemala-Mexico border is comparatively porous, and that condition is no more evident than along the stretch of the Suchiate that separates Ciudad Tecún Umán from the Mexican town of Ciudad Hidalgo.

Most people — perhaps thousands a day — sidestep the official border crossing between the two towns and instead travel by way of the river. Some are undocumented migrants heading north. Others are day laborers. Most, though, are people looking to buy or sell merchandise — mostly food, clothes and household supplies — without having to run the customs and immigration gauntlet.