Ice near shore is weaker. The buckling action of the lake or stream over the winter breaks and refreezes ice continually along the shore. Ice formed over flowing water can be dangerous near shore, around inflowing or outflowing streams, or on lakes containing large numbers of springs. River ice is generally about 15 percent weaker than ice on lakes. Straight, smooth flowing stretches are safer than river bends.
River mouths are dangerous because the current undermines the ice and creates unsafe pockets.
A potential danger spot on lakes is an open portion completely surrounded by ice. Winds will force exposed water beneath the ice and rot it from below. Other factors which weaken ice are water level fluctuations and the actions of birds and fish. As an example, schools of carp create thin ice spots or even open water by congregating in one location while circulating the water with their fins. Once you understand the physical properties and problems with ice, you can understand why ice is so unpredictable and why the only absolute safety factor on ice is to stay off.