Sustained low interest rates are starting to take their toll on banks' deposit inflows. Yesterday's announcement by the Reserve Bank that it would leave the cash rate unchanged at 2.5 per cent means that rates have not changed since August last year.Savers are starting to look for something else to do with their money besides putting it on deposit.According to Australian Prudential Regulation Authority statistics, total bank retail deposits increased by 9.9 per cent over the 12 months to April. Banks' mortgage portfolios grew by 6.3 per cent over the same period. Banks have been able to grow their mortgage books with the knowledge that deposits were increasing at a faster rate, keeping their target funding ratios intact.Things have changed in recent months. During the three months to April total bank retail deposits grew by 1.6 per cent - the same rate as the growth in banks' mortgage portfolios. Lenders can no longer rely on fast-growing deposit books to keep their deposit funding ratios where they would like them.Among the major banks, the ratio of deposits to total gross loans fell from an average of 75.5 per cent in the second half of the 2012/13 financial year to 74 per cent in the first half of the current financial year.Figures from the Reserve Bank of Australia tell a similar story. According to the RBA, term deposit balances grew steadily from the financial crisis before peaking at A$546.9 billion in October last year. Since then they have fallen 1.6 per cent to $538.2 billion in March.Deposits with non-bank authorised deposit-taking institutions have fallen by five per cent over the same period.