Water Quantity Limitations 
Climate and hydrology are natural limitations for water quantity. Most of the Pacific Northwest has wet winters and warm, dry summers. Precipitation is the primary water source during winter, but summer flow requires the release of stored water from snowmelt, groundwater, lakes and wetlands. Groundwater can provide a large percentage of annual flow (~60% in NW streams), making groundwater recharge necessary for sustaining summer flows.

Areas with modified hydrology, such as developed areas with impervious structures, prevent the recharge of groundwater. Impervious pavement and houses do not permit precipitation to infiltrate into the groundwater system, a process that occurs naturally in undeveloped areas. Water quantity can also be altered as water is withdrawn from groundwater and directly from streams and rivers for purposes such as growing crops. 

According to Waterwatch, Oregon withdraws over 8 billion gallons of water per day for all of its uses. This amounts to 3,000 gallons per day for every person in the state, which is the 7th highest water usage rate in the nation. Irrigated agriculture accounts for over 80% of Oregon's water use. Crops require more water in the summer and early fall when flows naturally run low.

Water Rights 
Water quantity and the right to use water is an issue worldwide. All water in Oregon is publicly owned according to state law. The Water Resources Department (WRD) can grant permits to extract water from any surface water or groundwater source. The WRD commonly issues "water rights" to farmers for agriculture, cities for drinking water, or as "in-stream water rights." An "in-stream water right" is the water quantity that fish and wildlife scientists feel is necessary to support the aquatic life inhabiting the waterbody. The basic tenets for water rights in Oregon are:

	Whoever received the earliest water right is first in line to get water if there is a limited water supply.
	The water use must be beneficial and without waste.
	The rights are attached to the land for which they are granted and therefore transfer when the land is sold.
	Water rights can be lost if not used in 5 years.

Most of the "in-stream water rights" established on behalf of fish are "junior" water rights, so in a drought or water shortage, all prior water rights would be honored first. In watersheds where the current water rights are causing summer stream flows that are critically low for fish, groups such as the Oregon Water Trust have been working to lease or purchase water rights for in-stream use.

For a more detailed introduction to Oregon's water rights laws, the following document is a great resource (PDF):
http://www.wrd.state.or.us/publication/pdfs/aquabook01.pdf.