watershed with null mean annual precip

[smnorris]

Null MAP values make the valley confinement script bail. This can be worked around and IIRC, null values can happen due to quirks in the processing where watersheds polygons are very small - and of course watersheds outside of the source raster will not have a MAP value.

Feature appearing to cause error is at mouth of Necleetsconnay River (Bella Coola), watershed_feature_id=10738401

This watershed looks like it should have a value, it may be worth investigating further - almost all of the feature is covered by mean_annual_precip.tif