The following should not be taken as legal advice, but simply as information based on general principles of law which is intended to educate. If you need legal advice, should consult a lawyer.
Washington has a legal doctrine referred to as the common enemy doctrine which allows landowners to alter the flow of water to the detriment of their neighbors. Washington courts have recognized this rule in some form for more than a century. The rule is that surface water, caused by the falling of rain or the melting of snow, and that escaping from running streams and rivers, is regarded as an outlaw and a common enemy against which anyone may defend themselves, even though by so they may cause injury to others. The landowner who causes damage to their neighbor is not responsible under this doctrine for the harm.
There are some exceptions to the common enemy doctrine. One provides that even though a downhill landowner can lawfully repel surface water from his or her land, it must be done without blocking a natural watercourse or drainway. A second exception provides that an uphill landowner cannot lawfully collect water in an artificial channel and then discharge it on adjoining lands. A third exception, is referred to as a "due care exception," and requires that the landowner act in good faith and with due care to avoid unnecessary damage to the property of others.
These cases are very fact specific and a lawyer experienced with this type of case should be consulted.