Sales grew at the U.S. National Whitewater Center but high expenses trimmed operating profit by 8 percent, to $4 million, according to the non-profit’s 2013 financial reports filed with Mecklenburg County. The figures include an upcoming $1 million subsidy from Mecklenburg County to the whitewater center — the final payment the county owes the center, the county park and recreation department confirmed Monday. Within the next month, the county will approve the annual $1 million payment for…