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"Financial institutions will need to start increasing interest rates on deposits in order to maintain current deposits levels and to increase liquidity ratio as mandated by Basel III," he added, referring to the international guidelines for bank capital requirements.
But the surge in balances over the past six years had been completely counter-intuitive from a return standpoint, in that deposit rates tumbled from 3.28 percent in July 2007 all the way down to 0.28 percent.
From the early stages of the financial crisis in mid-2007, deposit balances increased some 40 percent, from $6.7 trillion to $9.4 trillion.
That trend had come, though, as jittery investors yanked money from riskier mutual funds and poured their savings into plain-vanilla checking and savings accounts as well as money market funds.
And money markets continue to attract cash.
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After seeing net redemptions in the first quarter, money markets drew $50 billion, or 1.1 percent of total assets, in the second quarter, Market Rates Insight said.
The year has featured an overall change in where retail investors have moved money.
Bond mutual funds have seen dramatic outflows, with the biggest beneficiary nondomestic mutual funds—European equity funds have taken in $12 billion over just the last two months—and blended funds that offer exposure to stocks and bonds.
U.S.-based funds, meanwhile, have been only modest beneficiaries of what some experts predicted would be a "Great Rotation" from bonds to stocks.
If the trend out of deposit accounts continues, it could pose a further dilemma for the Federal Reserve, which has been repressing interest rates in hopes of stimulating economic development and investor risk-taking.
Rates have surged since the central bank began indicating that it likely will curtail its $85 billion a month bond-buying program.
While the Fed itself has not raised its targeted policy rate, the market has pushed yields higher on its own, and could continue to do so if banks have to raise deposit rates.
That would make its hopes to begin pulling back on quantitative easing more complicated, and pose a whole new set of dynamics for financial markets.
—By CNBC's Jeff Cox. Follow him @JeffCoxCNBCcom on Twitter.