The Federal Reserve sets interest rates, but its grip on the broader money supply runs through a less visible channel: the reserves that banks hold at the central bank. Understanding how that system works — and where it has limits — matters for anyone watching monetary policy today.
When the Federal Reserve raises or lowers its benchmark interest rate, the headlines tend to focus on borrowing costs for mortgages, car loans, and credit cards. But the deeper mechanics of how the Fed steers the economy run through bank reserves — the funds that commercial banks park at the Fed overnight.
Reserves are the raw material of the banking system. Banks are required to keep a minimum level of them, and the Fed pays interest on any reserves held above that floor. By adjusting that interest rate — known as the interest on reserve balances, or IORB — the Fed nudges the entire short-term lending market up or down. When reserves are plentiful, banks have little reason to lend them to each other cheaply, so rates stay anchored near the Fed’s target range.
Since the 2008 financial crisis, the Fed moved from a system of scarce reserves to one of ample reserves. It flooded the banking system with liquidity through large-scale asset purchases, commonly called quantitative easing. That shift changed how the Fed transmits policy. Rather than fine-tuning a small pool of reserves, it now operates with a large buffer — which gives it more control over short-term rates but complicates how that signal travels through to the real economy.
The limits of that control show up most clearly in broad money creation. Banks create money when they make loans, not when the Fed adds reserves. A bank flush with reserves will still only lend if it sees creditworthy borrowers and acceptable risk. So reserve levels and broad money growth can diverge — as they did during the pandemic era, when bank reserves surged but business and consumer lending did not keep pace at first.
As the Fed continues its effort to reduce its balance sheet — a process known as quantitative tightening — the level of reserves in the system is falling. Policymakers have said they want to keep reserves “ample” enough to maintain rate control, but determining that floor in real time is not straightforward. If reserves fall too far, short-term funding markets can seize up, as briefly happened in September 2019.
Watching reserve levels and the Fed’s balance sheet alongside rate decisions gives a fuller picture of where monetary policy actually stands.
