Conductivity measures the concentration of ions (charged particles) present in water. Conductivity is related to salinity, which measures just dissolved salts. Freshwater animals prefer lower conductivity, preferably under 500 μmhos/cm, but in San Pablo Creek we typically see upwards of 1,000 μmhos/cm. Conductivity may be related to the underlying geology of the stream bed, or result from urban runoff containing nutrients like phosphates and nitrates, which are found in fertilizers.
What can you do? Try not to over-fertilize plants, especially before it rains.