The report, released yesterday, is here.
From page 15:
Treasury estimates that China intervened heavily in the foreign exchange market to prevent a more rapid RMB depreciation. Treasury estimates that from August 2015 through February 2017, China sold around $800 billion in foreign currency assets to prevent rapid RMB depreciation. The pace of net foreign exchange sales appears to have abated somewhat in early 2017 amid strengthened enforcement of existing capital controls and stronger economic activity indicators.
Here’s a key graph from the report.