A site with suitable topography can save money during construction and avoid problems in operation. A natural draw or low area with a moderate slope which narrows at the dam site involves less earth moving and still provides adequate amounts of deep water. Extensive flat areas usually provide too much shallow water, which encourages the growth of aquatic vegetation and results in excessive evaporation losses. Extremely steep sides often contain too little shallow water for fish spawning to occur and may be unstable and slump into the pond. Major drainages should be avoided as pond sites because large amounts of runoff are difficult to retain. Damming a stream usually required an extensive spillway and modified dam to handle the volume of the flow. Streams also commonly carry high silt loads and provide access to the pond for undesirable fish species.