Rising water surface elevation without any change in flow

Hi all, After I added a second waterbody to the main stem/first waterbody in the model grid, I am having consistently increasing water surface elevation in the second waterbody when there is (almost) no flow there. Here are the details: 1. The model was running with no problem with the original, first waterbody for the whole model year (JDAYS1.0-366.0). 2. At its upstream end, second waterbody is tied to the original/first waterbody (Seg.20) with the pump. Pump starts operating on JDAY120.0. For the whole model year, inflow (with fixed 0.001 cms throughout the year) flow, temperature and water quality input files were assigned for the second waterbody. 3. At its downstream end, second waterbody is tied to the first waterbody (Seg.50) with no hydraulic structure ( it “naturally” joins the main stem/waterbody 1). However, EBOT for waterbody 2 was set to a higher value (than the one for waterbody 1) so that no flow will occur from waterbody 1 to waterbody 2 when there is no flow in waterbody 2. 4. There is “0.0” slope for both waterbodies. Even if the original slope is changed from 0.0 to very very small value for waterbody 2, the model does not start running. 5. Initial water surface elevation for waterbody 2 was set accordingly in the new bathymetry file which was created for waterbody 2. 6. There is no tributary (distributed or point) assigned for waterbody 2. The model starts running with the setup/details listed above. However, water surface elevation for waterbody 2 rises slowly and consistently until it overflows the model grid and the model crashes (when there is only “0.001 cms” flow rate through that period) before JDAY 121.0 when the pump starts operating. Any thoughts? Ertugrul (Ert)