Determine water level difference in two ponds I live near two ponds whose water levels appear to differ by a few feet. The ground is hard clay, so I don't think there's any underground water exchange between the ponds. The ponds are separated by about 30 feet of a bumpy terrain, with the bumps reaching a few feet above the "higher" water level. What is the cleverest way to determine the difference in water levels between these ponds? What comes to mind is to stick two poles (several feet high) at the water line in each pond, stretch a string between them, level the string with a bubble level and measure the distances between the string and the water level on each pole. Any other ideas?
 A: Fill a garden hose with water. Hold both ends closed, and walk to the "higher" pond. Have someone helping you hold the end of the hose under water.
Now walk to the other pond (still holding the end of the hose shut). Hold the hose near the surface of the pond - you should feel water pressing against your finger. Make a very small opening and observe the water coming out.
Now raise the hose slowly. At the point where the water stops flowing, the end of your hose is at the level of the higher pond.
Measuring the height difference is now trivial.
Incidentally - you mentioned that the ground between the two ponds was "bumpy". This raises the question of whether this bumpiness affects the result.
What we have here is a siphon: if the water is not flowing, the pressure at any point along the hose is simply given by the height of the point relative to the level of the body of water that the inlet of the hose is submerged in. In fact, the pressure will be
$$p = p_0 - \rho g h$$
Where $\rho$ is the density of water (usually 1000 kg/m3), $g$ is the gravitational acceleration (9.8 m/s2) and $h$ is the height above the pond level. As you can see, when $h$ becomes greater than 10 m the pressure will become negative: this means the water will boil (evaporate), creating a bubble in the hose. At that point, the relationship is no longer simple. On the other hand, if the hose goes through a valley, the local pressure will be higher - but this will not matter in the end.
In other words, this setup would work fine:

but this one would give you a problem:

Key to this working properly is to make sure that you don't let air flow back into the hose - hence the advice to only open the end of the hose by a tiny amount, and making sure that the other end is fully submerged in the higher pond. Note that since you end up doing a "null" measurement (where water stops moving), the length of the hose does not play at all in the answer.
The Egyptians did similar things to get the base of the pyramids level... except they filled a ditch with water all around the base, and made sure they were working off the (constant level) surface as their reference.
A: Buy  or rent a laser level, the kind that sits on a tripod. Place where it can "see" both ponds.  A self leveling one will be easiest to use.
Working at twilight, poke a stick into the mud of each pond, site it with the laser, and measure the height between the laser dot on the stick and the water level.
Repeat with the laser level in a different spot.
