Federal law enforcement agencies have struggled to reduce the amount of marijuana smuggled into the country through the U.S.-Mexico border while the Rio Grande poses as its biggest challenge.

According to USA Today, Arizona still leads the border states in the amount of volume the U.S. Border Patrol and Customs and Border Protection seizes there. Last year, the agencies captured more than 3.5 million pounds of marijuana.

However, both local officials in Texas and federal agencies estimate that, for every shipment they intercept, 10 more get through the winding Rio Grande Valley.

Border Patrol seized 797,000 pounds of marijuana in the Rio Grande Valley sector last year, while the Tucson sector confiscated 1.2 million pounds.

The drug cartels have made the Rio Grande one of the busiest passageways for marijuana smuggling and its members have gotten exceptionally good at studying the law enforcement's operations and patterns, Today reported.

The cartels have studied the Border Patrol's tactics by closely monitoring them. Their use of money and corruption as well as their ability to adapt quickly to many situations when facing law enforcement are how the loads of marijuana have still gotten through.

One way smugglers have learned to adapt is by following school bus routes when followed by authorities because police won't conduct a stop with children around, said Nat Gonzales, an investigator for a multiagency drug task for in Starr County, Texas.

"They take advantage of school traffic," he said. "They know we won't initiate a stop when there are students around."

The smugglers are also known to stop at the Rio Grande when followed by law enforcement, get out of the vehicle and escape across the river, leaving multiple pounds of marijuana behind.

According to investigations by the Arizona Republic, it was revealed that nearly every drug-smuggling case involving Border Patrol agents and the seizure of marijuana resulted in the use of force including firearms, physical force, less-lethal weapons and vehicle stopping devices.

The Republic pored through 12,000 pages of CBP and Border Patrol use-of-force incident reports during a 29-month period from 2010 to mid-2012, Today reported.

The investigation also revealed that there were only two incidents of involving the use-of-force and the smuggling of hard drugs including methamphetamine, heroin and cocaine between the same period.