A polder is a low-lying tract of land enclosed by embankments (barriers) known as dikes, that forms an artificial hydrological entity, meaning it has no connection with outside water other than through manually operated devices. There are three types of polder:

The ground level in drained marshes subsides over time and thus all polders will eventually be below the surrounding water level some or all of the time. Water enters the low-lying polder through ground swell due to water pressure on ground water or rain fall and transportation of water by rivers and canals. This usually means that the polder has an excess of water that needs to be pumped out or drained by opening sluices at low tide. However, care must be taken in not setting the internal water level too low. Polder land made up of peat (former marshland) will show accelerated compression due to the peat decomposing in dry conditions.

Polders are at risk from flooding at all times and care must be taken to protect the surrounding dikes. Dikes are mostly built using locally available materials and each has its own risk factor: sand is prone to collapse due to oversaturation by water while dry peat is lighter than water, making the barrier potentially unstable in very dry seasons. Some animals dig tunnels in the barrier, undermining the structure; the muskrat is notorious for this behavior. For this reason in the Netherlands it is actively hunted to extinction. No such care is taken in neighboring Germany though, causing the stock to be constantly resupplied across the border.^{[citation needed]}