Comment on this statement by an official of the Federal Reserve:

[the Fed] can control nonborrowed reserves through open market operations [but] it cannot control total reserves, because the level of borrowing at the discount window is determined in the short run by the preferences of depository institutions.

Quoted from Alfred Broaddus, A Primer on the Fed, Chapter 7 in Sumner N. Levine (ed.), The Financial Analysts Handbook (Homewood, IL: Dow Jones-Irwin, 1988), p. 194.