Over　lake　shores,　altimetric　waveforms　are　generally　contaminated　by　lands,　rough　lake　surfaces,　and　lag　effects　of　the　altimeter＇s　automatic　gain　control.　To　improve　altimeter　ranging　accuracy　and　in　turn　to　get　better　surface　height　measurement,　contaminated　waveforms　should　be　retracked　against　geophysical　corrections.　In　this　paper,　an　improved　threshold　retracker　(ITR)　is　developed　to　retrack　waveforms　over　lakes.　ITR　considers　not　only　the　physical　characteristics　of　the　reflecting　surface,　but　also　the　stochastic　feature　of　waveform,　and　two　new　retrackers,　the　N-Beta　function　model,　and　the　N-5-Beta　function　model,　are　also　put　forward　to　develop　the　waveform　retracking　program　of　this　study.　TOPEX/POSEIDON　waveforms　over　Hulun　Lake　in　the　North　China　are　retracked　to　monitor　the　temporal　lake　level　variations.　A　comparison　with　the　in　situ　hydrological　data　indicates　ITR　is　very　efficient　to　monitor　the　lake　level　variations　with　the　retracked　altimetric　data.　The　result　of　our　study　shows　accurate　seasonal　level　variations　and　the　descending　trend　of　Hulun　Lake.　©　2009,　The　Institute　of　Electrical　and　Electronics　Engineers,　Inc.