I could be wrong, but I think this is the area for the rivers that you want to modify to change tile yields. Similar spot for the forest files.
-<GameModifier InternalName="CityTileYield_River_Grain">
<ModType>Resource</ModType>
<Attribute>TileYieldGrain</Attribute>
<Value>1.5</Value>
<PerTurn>1</PerTurn>
</GameModifier>
-<GameModifier InternalName="CityTileYield_River_Materials">
<ModType>Resource</ModType>
<Attribute>TileYieldMaterials</Attribute>
<Value>1.5</Value>
<PerTurn>1</PerTurn>
</GameModifier>