Whatever their design, fishways are all built so as to assure the alewives quick transit from one pool to another. In most cases they seem to work satisfactorily, but unless they are well managed they can effect full use of the stream by the migrant fish. Alewives have their own crowd pressure and motion, their way of moving on, and any reduction in their numbers at any one time or counterpressure, keeping them back, may result in a decline because less will get to the ponds to spawn. Fishways are more rigid than a natural stream bed, though sometimes less hazardous. Water levels change; the flow varies both in angle and pressure; and managers of a good fishway must be constantly on the alert for new conditions. A marked increase or decrease in the volume of water, especially as it is reflected at the head of a fish ladder, which usually has a gate or wooden dam of some kind, may create a barrier instead of an aid. Unless the adjustment in the dam is just right the head of water coming down may be almost impossible for a fish to surmount. Alewives are not like the muscular west coast salmon with their spectacular leaps, as if shot by a giant sling. An alewife does not leap over a pool and up a falls so much as swim through it rapidly, being a much smaller fish and in smaller streams. If a head of water coming over a dam or sluice is at the wrong pitch, the fish will not be able to climb it. In designing a fishway an engineer has to take into account the relationship between the water head and the angle of the flow below it, which has to be translated into how far an alewife can swim at what speed. A fish is supported by water--an advantage over cumbersome human beings in their own surrounding medium, the air--its specific gravity being close to that of its own body. The fish is so made as to swim through the water with as little resistance as possible. It also gets energy from the water, orienting itself by the current, or the various changes of pressure in the flow, the way a bird uses currents in the air. In so far as a bird is streamlined too, and finds in air pressure and weight the means to fly up and forward, their actions have something in common. A bird, like an alewife, may lose its momentum if the angle of climb gets too steep. Swimming and flying take place in fluid surroundings. The difficulty in making an artificial aid like a fishway comes from the problem, in part, of understanding a fish’s behavior; of meeting its needs; arranging its passage; trying, if not to control nature, at least to be a substitute for it. The positive results are plain to see, but there is something almost as elusive about it as trying to explain a fish in human language.