To add onto Peter M's answer (where Civil Asset Forfeiture is mentioned in passing), it should be noted that you have far fewer rights with CBP than you would with police. An Albanian man attempted to fly with $58,100 that wound up seized.
It all began last October, when CBP agents at Cleveland Hopkins International Airport seized $58,100 from Rustem Kazazi, a U.S. citizen, while he was headed to a layover in Newark during a trip back to his native Albania. Rustem, a former Albanian police officer, had worked hard to save up the money with help from his wife, Lejla, and son, Erald, over a dozen years after the family immigrated to Ohio in 2005. Before Rustem could board the plane, CBP agents strip-searched him, interrogated him without a translator and then took his family’s entire life savings, even though they never found anything illegal. They have held the family’s savings for more than seven months, despite never charging anybody with a crime.
Oh, and CBP counted less money than he had on him
Compounding insult and injury, the receipt CBP agents gave Rustem at the time did not note the amount of the seized “U.S. Currency.” CBP later claimed to have taken only $57,330—$770 less than he was actually carrying.
Why did they seize the money? They claim he was involved in drug trafficking
“This is to notify you that Homeland Security Investigations (HSI) seized the property described below at Cleveland, OH on October 24, 2017: $57,330 in U.S. Currency,” the notice states. “Enforcement activity indicates that the currency was involved in a smuggling/drug trafficking/money laundering operation.”
They also accused him of a money laundering crime: structuring. That's where you move money in increments designed to avoid Federal reporting requirements.
In a statement, CBP said that “pursuant to an administrative search of Mr. Kazazi and his bags, TSA agents discovered artfully concealed U.S. currency. Mr. Kazazi provided inconsistent statements regarding the currency, had no verifiable source of income and possessed evidence of structuring activity,” that is, making cash withdrawals of less than $10,000 to avoid reporting requirements.
If I were you, I would not travel with that much cash across the border, declared or otherwise. CBP has immense search power.