Pond Boss Forums POND CONSTRUCTION Building a dam Cinder-block dam with reinforced rebar?

Your plan suggests problems with as noted by Fishinrod that eventual lateral pressure cracking of the joints will occur. A liner placed on the inside of the wall could help reduce seepage from eventual joint cracking. My other concern is water pressure and movement can occur around the outside edge of the dam block structure and the eventual pressure and seepage will cause a gradual water movement pathway to occur round the outside of the wall. This is more likely to occur if there are any porus soils on the outside edges of the dam. Seepage will gradually continue to increrase over time. Lots of hydraulic pressure will occur at 9-10 ft deep to induce and gradually and slowly produce a seepage pathway that will over time increase to a significant flow rate. If long term structure of the dam block structure is important, I would consult with a soil engineer and or similar construction professional. Some government agencies (US Soil & Water? or Univ Extension Service) have soil engineers on staff or know about them for a free consult. Two professional opinions would be wise.