Question
if a row or line of conifer trees are planted can this stop water or dam water from the ground or land behinde them ?

Answer
If you are asking if a line of conifer trees is planted on a hill side will it block the water from going below the line the answer is NO. The root systems are usually in the upper tow feet of soil except for pines that have a tap root that goes deeper. They are not a solid mass of roots but individual roots with soil around them so the water will continue through any root system on a slope. The water will not be stopped or dammed up by the roots.